Shift+Delete not deleting clips

The Shift+Insert and Shift+Delete are great features to quickly modify the song's arrangement.

However I'd expect the Shift+Delete function to actually **delete** the clips it eats. Instead, it piles them right behind the playhead and creates a nasty mess that the user has to clean up manually with a minute or two of middlemouse-clicking to remove the clips.

With the new undo funcions (YEEEEES!! THANK YOU DEVS!!! xD xD) the clip deleteion should be not a problem to bring them back (_should_).

Start by locating the arrangement action handling Shift+Delete and compare its behavior with Shift+Insert. Reproduce the issue with clips around the playhead, then verify that Shift+Delete removes the affected clips and that undo restores them without leaving extra clips behind.

Written by the indexing model from the issue text.