Move all beats <-> frames time conversion into a single object that can be passed...

Move all beats <-> frames time conversion into a single object that can be passed around.

This has 3 main benefits:
 - All conversion code is in one place (less duplication, potential bugs)
 - The conversion method can be passed to things that are ignorant
   of the actual time units involved, information required, etc.
   (In the future it would be nice to have user selectable tempo/frame time)
 - It should be relatively simple now to support tempo changes part-way
   through a MIDI region (at least architecturally speaking)

git-svn-id: svn://localhost/ardour2/branches/3.0@4594 d708f5d6-7413-0410-9779-e7cbd77b26cf