Allow plugins to contribute to in-app object selection.

Also, move the widget selection from the probe into the widget plugin.
This removes yet another QApplication dependency from the probe.
Move model selection from QAbstractItemViews to the widget plugin.

This removes the QtWidgets dependency from the model inspector.
Make QApplication detection work without linking against Qt::Widgets.

This removes the last widget dependency from the probe, only a few in the
resource model left now.