Make DebugView great

Fix up https://codereview.qt-project.org/c/qt/qtquick3d/+/438466 to get a first class DebugView that uses real Controls with models with the mesh and texture tables.

As we are planning to add other developer functionality to this view, i.e. to trigger (re)baking of lightmaps or reflection probes (QTBUG-109815), it would be ideal if the view gets fixed and made nice first.

With the patch, we now get this:

This view is great.

But the item views have lots of sizing-related and usability issues:

Note: have to watch out for other styles (the native ones) as well, in case an application is using those.