Import/export MIDI patterns like is done with .xpts

### Enhancement Summary
Ability to import/export MIDI patterns. In PR 5891, the ability to import/export patterns in the form of .xpt (LMMS-readable xml pattern file) was introduced.
Request for ability to import and export MIDI patterns too. When exporting, option for "MIDI" should be there along with .xpt option under "Files of type" option.

#### Justification
General convenience and compatibility with non-LMMS platforms required. Personally, it would be of use to me because I use an external application for humanizing, and it accepts only MIDI files.

Start by reviewing PR 5891 and the existing .xpt pattern import/export implementation. Trace how the export “Files of type” option is defined, then identify the corresponding MIDI import and export paths. Done means MIDI is available alongside .xpt and patterns can be exchanged with external MIDI applications.

Written by the indexing model from the issue text.