Port 'Cut time' code from Mixbus

Port 'Cut time' code from Mixbus

Copy the 'Cut time' code from Mixbus, making a few obvious fixes to work in
A3 (e.g. nframes_t => framepos_t / framecnt_t).

Seems to work to move & remove markers, tempo & meter markers, and regions on
selected tracks.

Still TODO:
 - use existing A3 'Insert time' dialogue
 - make it respect 'No selection = all tracks'
 - rename the command to something like 'Remove time' or 'Delete time': 'Cut'
   sounds to me as if the removed range should end up on the clipboard ready
   to be pasted somewhere, which of course it doesn't.