Add anchors property to Popup to allow centering in parent/window

Currently, users must manually position their Popup using x and y
bindings:

    Popup {
        x: (parent.width - width) / 2
        y: (parent.height - height) / 2
    }

This patch adds an anchors property so that you can do this instead:

    Popup {
        anchors.centerIn: parent
    }

It's also possible to conveniently center within the window from
anywhere within the scene (106e7b63 also documents an alternative way
of doing this using Overlay):

    Window {
        id: window

        Pane {
            Popup {
                anchors.centerIn: window
            }
        }
    }

QQuickAnchors were never used with Popup, because we cannot use the
QQuickAnchors implementation as-is, as the visual QQuickItem parent
is not the actual parent item of QQuickPopupItem.

Currently just centerIn is supported, as that's the most common
use case.

[ChangeLog][Controls][Popup] Added anchors.centerIn to Popup to allow
a covenient way of centering a popup.

Task-number: QTBUG-60354
Change-Id: Ia030f812df9da646fea8f373ef6199a21205ffbd
Reviewed-by: J-P Nurmi <jpnurmi@qt.io>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/quicktemplates2/quicktemplates2.pri b/src/quicktemplates2/quicktemplates2.pri
index c37526d4..425b3c8f 100644
--- a/src/quicktemplates2/quicktemplates2.pri
+++ b/src/quicktemplates2/quicktemplates2.pri
@@ -55,6 +55,8 @@ HEADERS += \
     $$PWD/qquickpane_p_p.h \
     $$PWD/qquickpopup_p.h \
     $$PWD/qquickpopup_p_p.h \
+    $$PWD/qquickpopupanchors_p.h \
+    $$PWD/qquickpopupanchors_p_p.h \
     $$PWD/qquickpopupitem_p_p.h \
     $$PWD/qquickpopuppositioner_p_p.h \
     $$PWD/qquickpresshandler_p_p.h \
@@ -131,6 +133,7 @@ SOURCES += \
     $$PWD/qquickpaletteprovider.cpp \
     $$PWD/qquickpane.cpp \
     $$PWD/qquickpopup.cpp \
+    $$PWD/qquickpopupanchors.cpp \
     $$PWD/qquickpopupitem.cpp \
     $$PWD/qquickpopuppositioner.cpp \
     $$PWD/qquickpresshandler.cpp \