Fix displaying selected Bezier animation keys in inspector

[CookieBadger]

fixes #93308 and other regressions regarding displaying selected bezier animation keys in the inspector.

Unfortunately, this fix was not as simple, as selecting keys was not handled correctly across almost all features of the bezier editor. For example, handling multiple selected keys in the inspector was implemented, but there were some bugs not allowing the feature to work or throw errors during editing.

With this fix, key selection should be consistent across the animation UI and the inspector with all operations (selecting with click/shift-click/box/shortcut, deselecting, inserting, copying/duplicating, etc.). I would appreciate any help testing if there is another way to get inspector and animation UI to disagree in their selection that I might have missed.

EDIT: just discovered this also fixes #29155, although a backport to 3.x might not be a bad idea.

[Calinou]

Tested locally, it works as expected.

[akien-mga]

Thanks!