Similar to QWidget::parentWidget(), but not tied to QWidgets.
Allows to start removing some asQWidget() calls, as seen in Frame::floatingWindow(),
did it for POC.
Still need to find a generic way to compare 2 QWidget pointers
Controllers don't have any GUI and don't depend on the frontend
technology.
Views are implemented in a specific GUI tech (QtWidgets, QtQuick, etc).
For now only QtWidgets work. There's still a lot to decouple.
This will make it easier to introduce non-Qt backends.