Before this, copy-pasting sequencer strips with animation on them would
fail to copy-paste their animation along with them if they were animated
by a slotted action. This fixes that.
There are three remaining known issues in the sequencer when used with
slotted actions that this PR does not fix, and will be addressed in a
follow-up PR:
1. Moving strips fails to move the animation of their fcurves with them,
which it should.
2. Duplicating strips (as distinct from copying followed by pasting)
fails to duplicate their animation with them.
3. Deleting a strip does not delete its animation channels with it.
Pull Request: https://projects.blender.org/blender/blender/pulls/128363
e34feb347c