+ Refactor: Cleans up code concerning Krita's Animation Timeline Docker
in order to minimize code duplication, generalize functions, and simplify some aspects of the interface for developers and users alike.

+ Bugfix: Fixes a regression caused by 7c639729 which caused "insert
frames left/right" and "insert columns left/right" to behave incorrectly
when multiple frame slots were selected.

~ Reviewed and corrected by Dmitry. Programmed with Eoin. Thanks as
always!