Remove bar doesn't preserve clip offsets

[Spekular]

Found while testing 5699, but turns out this happens in 1.2.2 and master builds as well.

Steps to reproduce

• Create clips with different start positions in the song editor
• Press "remove bar" (or shift+ delete) until at least one clip is less than a bar away from the start of the song
• Continue pressing "remove bar"

Expected behavior

• "Remove bar" should only move clips if all clips can be moved left by a bar

Actual behavior

• Different clips may move different distances

Screenshot

Here "remove bar" has been pressed repeatedly:

Affected LMMS versions

1.2, master