Enhance editor performance

Larger scenes suffer from the many text elements in the tree graphs and property editor. The scenes itself are good as soon as all the labels have been loaded.

Some ideas:
Load only visible labels and some that may be visible soon (lazy loading)
Pack labels in the scene graph into one node and flatten them, make sure buttons and other clickable elements won't be affected or labels misplaced.

No files or tests are named. Start by profiling large scenes in the tree graphs and property editor, focusing on the many text labels and their loading cost. Done means large scenes load or remain responsive while labels stay correctly positioned and buttons and other clickable elements continue to work.

Written by the indexing model from the issue text.