Allow collective stretching of multiple clips in Song Editor

Nobody has claimed this yet.

### Enhancement Summary

I'm an LMMS user and truly grateful for this free and open-source software.

Today I was working in the Song Editor and selected several clips (both audio or MIDI sequences) to stretch them longer simultaneously, but I found that this action is currently not supported.

I believe adding a collective length scaling feature would significantly improve the user experience. It would make batch editing much more efficient and bring the Song Editor's behavior more in line with the Piano Roll, where similar functionality already exists.

### Implementation Details / Mockup

In the Piano Roll, multiple notes can already be selected and scaled together. I believe the same logic could naturally be extended to clips in the Song Editor. When multiple clips are selected, dragging the edge of any one of them should scale all selected clips proportionally.

This would provide a consistent and intuitive workflow across different editing contexts within LMMS.
(lf the function is surplus, neglect me)

### Please search the issue tracker for existing feature requests before submitting your own.

- [x] I have searched all existing issues and confirmed that this is not a duplicate.

Start by examining the existing multi-note scaling behavior in the Piano Roll, then trace how clip selection and edge dragging work in the Song Editor. Done means selected audio and MIDI clips can be proportionally length-scaled together by dragging one selected clip's edge, with behavior consistent with the Piano Roll.

Written by the indexing model from the issue text.