Add a callback mechanism to run custom setup logic on all framework-created QQuickWindow instances

[maxime-modulopi]

530e045 added a way to do this when new FloatingWindow instances are created.

Unfortunately, it currently cannot be done for other framework-created windows, such as the IndicatorWindow. This has the effect of Qt creating new QRhi instances (and new VkInstance + VkDevice) when new QQuickWindow instances are created, which may confuse the Vulkan Validation Layers and other similar debugging tools.

To remedy that, we would need a similar callback mechanism, but for all QQuickWindow instances created by KDDockWidgets.

[maxime-modulopi]

Thank you!