Add QMessageBox backend for MessageDialog

Tobias Fellarequested to merge
work/tobias/messagedialog into master

Make the Platform theme provide a native implementation of MessageDialog. For widgets apps, this effectively shouldn't change anything, since the implementation uses the same component; for QML apps using MessageDialog, the QMessageBox widget is now used as implementation instead of a qt-provided qml fallback, which is a significant visual improvement:

before:

image

after:

image

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