gui: add WaiterOverlay. To display processing wheel on top of another widget #1230

Job	Run time
Python 3.7 on ubuntu-20.04 PyQt5 sdist	6m 50s
Python 3.10 on macos-latest PyQt5 wheel	10m 5s
Python 3.8 on ubuntu-latest PySide6 sdist	7m 56s
Python 3.9 on macos-latest PySide6 wheel	14m 22s
Python 3.7 on ubuntu-latest PyQt6 wheel	7m 12s
Python 3.10 on macos-latest PyQt6 wheel	14m 35s
Python 3.9 on windows-latest No GUI wheel	9m 41s
	1h 10m 41s

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