Blender Git Loki

Fix T82364: Widget anim state not updated when deleting channel in Graph Editor

So a keyframed e.g. location slider would stay yellow/green even if its
corresponding channel was removed.

Needs a appropriate notifier so the listeners (e.g.
buttons_area_listener, view3d_buttons_region_listener) would cause a
redraw.

Maniphest Tasks: T82364

Differential Revision: https://developer.blender.org/D9438