Move time converters only to the region views that actually need them.

A step towards sorting out time issues more solidly, the time situation of MIDI
region views and automation region views is slightly different.
Factor out copy-paste code to get grid type in beats defaulting to 1.
Fix const violation warnings in tempo.cc.
Fix assert failure in control list interpolation.

How we never hit this before is beyond me, it's in some ooooold code.