-Changed Animation to have a special signal when tracks are changed, to avoid unnecesary track cache rebuilds in AnimationPlayer
-Added missing emit_changed whe modifying keys to Animation
-Changed AnimationPlayer to use the new refcounted connections instead of the previous hacky way to keep references
-Changed AnimationEditor to update the current track when keys are edited
-Fixed bug where undo/redo did not work with AnimationKeyEdit (was not being updated)
-Made sure UndoRedo does not mind deleted objects in undo/redo history, this would corrupt the history or clear it without need.
Found via `codespell -q 3 --skip="./thirdparty,./editor/translations" -I ../godot-word-whitelist.txt`
Whitelist consists of:
```
ang
doubleclick
lod
nd
que
te
unselect
```
Using `misc/scripts/fix_headers.py` on all Godot files.
Some missing header guards were added, and the header inclusion order
was fixed in the Bullet module.
