CMake: Add convenience custom targets to build Qt plugins

Add 3 new convenience custom targets:
'qt_plugins', 'qpa_plugins' and 'qpa_default_plugins'.

Additionally, if we detect that an internal executable / test
links against Gui, add a dependency on the 'qpa_default_plugins'
custom target, so that if a developer configures Qt for the first time
and then calls ninja 'tst_foo_check', we ensure the test will launch
successfully because the default QPA plugin will also be built.

Change-Id: If6dd70844b5effdf8a293f65f8785855cc85b132
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

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