Verify that drag and drop between widgets and Qt Quick works

Axel have done some preliminary tests with dragging and dropping between widgets and QtQuick. It looks mostly ok, but we should take a bit deeper look into it, and also add drag and drop to the example.

Subtasks:

1. Verify that DnD between widgets <-> Qt Quick works (both ways) by adding it to the embeddedinwidget example. In addition, we might have to add manualtests in order to explore this properly.

1. If we identify issues with DnD between widgets and Quick, we should try to fix those issues.