When copying strips between 2 scenes, it wasn't possible to copy
animation curves along with strips.
In this patch curves are copied into clipboard `ListBase`. When pasted,
original curves are moved into temporary `ListBase` and curves in
clipboard are moved into scene action. This is because when strips from
clipboard have to be renamed, function `SEQ_ensure_unique_name()` does
fix RNA paths of curves, but this is done globally for all curves within
action. After strips are renamed, restore original curves from backup.
Note: This patch handles only fcurves. Drivers and actions are currently
not handled anywhere in VSE.
Fixes T77530
Reviewed By: sergey
Differential Revision: https://developer.blender.org/D13845
eddad4e9a1