Qt6 WIP

[mjcarroll]

Toward #586

This is in no way ready, but just to be publicly accountable for progress ;)

[caguero]

Plugin status after 1f28e07: This list evaluates whether the plugin loads or not. It doesn't check its full functionality.

• MarkerManager
• NavSatMap:  It doesn't load. Can't find module QtLocation. It seems that his module is available starting from Qt 6.5.x
• TopicEcho
• TransportPlotting
• Screenshot
• WorldControl
• TransportSceneManager
• ShutdownButton
• Teleop
• CameraTracking
• KeyPublisher
• TapeMeasure
• TopicViewer:  Loads commenting many of the properties of TreeView, including delegates.
• CameraFps
• Publisher
• ImageDisplay: Loads OK but I don't see the list of topics updated.
• PointCloud
• GridConfig: Loads but requires an updated version of GzSpinBox to be functional
• WorldStats
• MinimalScene:  Loads OK passing a config file with the engine name, etc.
• InteractiveViewControl

[jennuine]

Posting here as a friendly reminder once we've finished Qt6 migration to test out seting the flags of its RHI texture to sRGB color space before we potentially port the #629 workaround.