Support the qApp macro correctly, final version incl. debug

For short the new features:
- there is a qApp in QtCore, QtGui and QtWidgets for compatibility,
  and also in __builtins__ for a true macro-like experience.
- if you delete any qApp variable, the Q*Application is reset and you can
  start over.

Long description:

There is a qApp macro in Qt5 which is equivalent to Q*Application.instance() .

Python does not have macros. Both PyQt5 and PySide2 have an
according structure in QtWidgets. In the case of PySide2, the qApp
variable is first initialized to None and later to QApplication().

This does not reflect the original sense of the qApp macro, because
 - it only handles QApplication,
 - it does not handle destruction.

This "macro" should live in QtCore, but both PyQt5 and PySide2 decided
to put this in QtWidgets. As a compromize, I propose to put qApp into
all three modules, and into __builtins__ as well, so wherever you
create an application, you find this "macro" in place.

While changing the code, I stumbled over the template
set_qapp_parent_for_orphan. I tried to make sense out of it and finally
removed it. There were no side effects but bug PYSIDE-85 is gone, now.

With some extra effort, I created a singleton qApp that changes itself.
This way, a true macro was simulated. Note that this was not possible
with a garbage collected variable, and I had to make shiboken aware of this.

As the final optimization, I turned qApp also into a fuse variable:
Delete any qApp variable and Q*Application will finish when there is
no extra reference.

Task-number: PYSIDE-85
Task-number: PYSIDE-571
Change-Id: I7a56b19858f63349c98b95778759a6a6de856938
Reviewed-by: Christian Tismer <tismer@stackless.com>

2 files changed, 77 insertions, 0 deletions

diff --git a/sources/pyside2/tests/pysidetest/CMakeLists.txt b/sources/pyside2/tests/pysidetest/CMakeLists.txt
index d46498f77..821b6f819 100644
--- a/sources/pyside2/tests/pysidetest/CMakeLists.txt
+++ b/sources/pyside2/tests/pysidetest/CMakeLists.txt
@@ -124,3 +124,4 @@ PYSIDE_TEST(mixin_signal_slots_test.py)
 PYSIDE_TEST(signal_slot_warning.py)
 PYSIDE_TEST(all_modules_load_test.py)
 PYSIDE_TEST(signature_test.py)
+PYSIDE_TEST(qapp_like_a_macro_test.py)
diff --git a/sources/pyside2/tests/pysidetest/qapp_like_a_macro_test.py b/sources/pyside2/tests/pysidetest/qapp_like_a_macro_test.py
new file mode 100644
index 000000000..6205748ec
--- /dev/null
+++ b/sources/pyside2/tests/pysidetest/qapp_like_a_macro_test.py
@@ -0,0 +1,76 @@
+#############################################################################
+##
+## Copyright (C) 2017 The Qt Company Ltd.
+## Contact: https://www.qt.io/licensing/
+##
+## This file is part of the test suite of PySide2.
+##
+## $QT_BEGIN_LICENSE:GPL-EXCEPT$
+## Commercial License Usage
+## Licensees holding valid commercial Qt licenses may use this file in
+## accordance with the commercial license agreement provided with the
+## Software or, alternatively, in accordance with the terms contained in
+## a written agreement between you and The Qt Company. For licensing terms
+## and conditions see https://www.qt.io/terms-conditions. For further
+## information use the contact form at https://www.qt.io/contact-us.
+##
+## GNU General Public License Usage
+## Alternatively, this file may be used under the terms of the GNU
+## General Public License version 3 as published by the Free Software
+## Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+## included in the packaging of this file. Please review the following
+## information to ensure the GNU General Public License requirements will
+## be met: https://www.gnu.org/licenses/gpl-3.0.html.
+##
+## $QT_END_LICENSE$
+##
+#############################################################################
+
+import unittest
+import PySide2
+
+# This test tests the new "macro" feature of qApp.
+# It also uses the qApp variable to finish the instance and start over.
+
+class qAppMacroTest(unittest.TestCase):
+    def test_qApp_is_like_a_macro_and_can_restart(self):
+        from PySide2 import QtCore
+        try:
+            from PySide2 import QtGui, QtWidgets
+        except ImportError:
+            QtWidgets = QtGui = QtCore
+        # qApp is in the builtins
+        qApp
+        # and also in certain PySide modules
+        QtCore.qApp, QtGui.qApp, QtWidgets.qApp
+        # and they are all the same
+        self.assertTrue(qApp is QtCore.qApp is QtGui.qApp is QtWidgets.qApp)
+        # and the type is NoneType, but it is not None (cannot work)
+        self.assertTrue(type(qApp) is type(None))
+        self.assertTrue(qApp is not None)
+        # now we create an application for all cases
+        classes = (QtCore.QCoreApplication,
+                   QtGui.QGuiApplication,
+                   QtWidgets.QApplication)
+        for klass in classes:
+            print("created", klass([]))
+            del __builtins__.qApp
+            print("deleted qApp")
+        # creating without deletion raises:
+        QtCore.QCoreApplication([])
+        with self.assertRaises(RuntimeError):
+            QtCore.QCoreApplication([])
+        # assigning qApp is obeyed
+        QtCore.qApp = 42
+        del __builtins__.qApp
+        self.assertEqual(QtCore.qApp, 42)
+        self.assertNotEqual(__builtins__, 42)
+        # delete it and it re-appears
+        del QtCore.qApp
+        QtCore.QCoreApplication([])
+        self.assertEqual(QtCore.QCoreApplication.instance(), QtCore.qApp)
+        # and they are again all the same
+        self.assertTrue(qApp is QtCore.qApp is QtGui.qApp is QtWidgets.qApp)
+
+if __name__ == '__main__':
+    unittest.main()