This adds an Animation panel to the Mask tab of the n-panel of the
Movie Clip and Image editors. Masks can be animated (for example, the
opacity of a Mask Layer), but there was no way to manage the Action
and Slot that held those F-Curves.
To keep things DRY, this PR also moves the code for drawing Action+Slot
selectors from the `PropertiesAnimationMixin` class to a utility
function, which is now called from both that class and the Mask UI code.
Pull Request: https://projects.blender.org/blender/blender/pulls/133153
b2a06888c7