Drag-and-drop for MIDI files in song editor

### Enhancement Summary

Currently, dragging a MIDI file into the song editor automatically creates a new instrument track for the MIDI clip and three automation tracks for the time signature and tempo data. However, I always end up moving the MIDI clip to the instrument I intended to use it with. Being able to directly place MIDI clips in any instrument track at any spot would be a great workflow enhancement for working with MIDI files.

### Implementation Details / Mockup

This feature would act the same way as dragging audio files onto sample tracks in the song editor. The additional automation tracks may or may not still be created in addition to the added clip (see #8257).

### 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 in the song editor's existing audio-file drag-and-drop behavior and compare it with the current MIDI import path, using issue #8257 to clarify whether automation tracks remain. Done means a MIDI clip can be dropped directly onto any instrument track at the intended position, with the automation behavior resolved.

Written by the indexing model from the issue text.