Implemented update-on-dirty for SceneInspector inheritance popup windows.

The SceneInspector class can now be instantiated with a specific set of Sections, and the target paths may be pinned. The inheritance diagram is now implemented as a section, meaning we can simply put a small SceneInspector in the popup window and take advantage of all the update machinery implemented there. We could also use the same mechanism to tear off copies of any SceneInspector section if we wanted to in the future.
Fixed SceneInspector inheritance window layout glitch.

I don't fully understand what is happening but I think it's because when we use the auto-parenting mechanism for a parent which is already visible, Qt realises the child QWidgets earlier in the process, and then fails to update the layout when we change their properties subsequently. By using `_mainColumn()[:] = rows` we not only avoid this, but also do the deletion of existing rows and addition of new ones in one go, which cuts down on intermediate updates and gives a snappier build.