MIDI API: get all editable items

[nofish]

Mentioned before (e.g. here) but not 'officially' requested yet afaik.

All these great MIDI scripts we have can currently only work on one active item*, so please add this API to make multi-MIDI items editing with scripts possible (as Reaper's native MIDI editing already can).

Thanks.

* edit:
or all or selected items, which might not be same as all editable items depending on MIDI editor settings.