This is a follow-up of D9499 which moves each remaining outliner tree display to subclasses of AbstractTreeDisplay.
The display modes classes in this patch are:
- Video Sequencer: TreeDisplaySequencer
- Orphan Data: TreeDisplayOrphanedData
- Scenes: TreeDisplayScenes
- Data API: TreeDisplayData
Some small cleanup is also done in outliner_tree.c now that all tree building code is in C++.