Remove bar doesn't preserve clip offsets

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

Start in the song editor by reproducing the issue with clips at different start positions, then repeatedly use “remove bar” or Shift+Delete as described. Done means the command moves clips left by a bar only when every clip can move that far, preventing different clips from shifting by different distances.

Written by the indexing model from the issue text.