Fix bumping .mid file name (snapshots & playlist copy)

Fix bumping .mid file name (snapshots & playlist copy)

When forking regions, copying playlists or saving snapshots we do not
have a reference to the track and cannot use the track's name as basis
for the new filename like Editor::fork_region() does.
A cloned midi region's name is based on the original region name.

This prevents endless addition "name-1-1-1-1-1-1-1-1.mid", adding
to the region's basename.