Add ability to loop MIDI clips

Nobody has claimed this yet.

### Enhancement Summary

I'd like to be able to loop MIDI clips. Beat&Bassline clips achieve this, But they require a separate track for even a mildly different clip, which means the only options available to the user if they want to repeat anything is to:

a) use a beat&bassline clip, but they'll need to create loads of tracks if they want loads of variation
b) use a MIDI clip and copy and paste it, which is slightly annoying as if they make any change they’ll need to delete all the other repetitions and paste again.

Being able to repeat MIDI clips meets these in the middle as if the user wants a slight variation in, for example, a melody for a certain section, they can just just have different looped sections on the same track.

### Implementation Details / Mockup

Personally, I like the way GarageBand (and presumably Logic Pro) handles this. It has a dual handle design, in which dragging from the top right of the clip initiates a loop, while dragging from the bottom right initiates a resize. An issue with this is that the default track height in LMMS is a bit too small for this, so we'd have to increase it. Honestly, though, that would be a good change IMO. I find LMMS a bit cramped.

Another way we could do this is to use a modifier key, e.g shift, to initiate a loop.

Idealy, this could also be used on samples, automations and maybe even b&b clips.

### 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 reviewing how MIDI clips currently behave in LMMS and compare the proposed GarageBand-style dual handles with the modifier-key alternative. The issue also suggests possible support for samples, automations, and Beat&Bassline clips; done would require an agreed interaction design and working looped MIDI clips without requiring repeated copies.

Written by the indexing model from the issue text.