Indicators not showing due to other `KDDockWidget` in background

[bbc131]

If you have two KDMainWindows overlaying each other, sometimes the drop-indicators are prevented from being displayed because of the other main window.

To reproduce, simply run the qtwidgets_dockwidgets example with the options -i -m to get two main-windows, being incompatible with each other. I tested it on the main branch commit 57b5e69d1c.

[bbc131]

I just figured, that this isn't related to the two main-windows or using affinities. I can reproduce it simply with three floating dockwidgets using the qtwidgets_dockwidgets without any options set.

[iamsergio]

Reproduced