Added new operator for point cache 'Export'. This is basically the same as baking, naming may have to be adjusted later, for now they just exist side-by-side.
Added a new DNA file DNA_pointcache_types.h for all point cache related
structs and enum values. DNA_object_force.h was use previously, this is
a badly named file with a hodgepodge of different areas.
New settings struct for poin caches (unused still).
Make sure the output directory for cache files exists before trying
to create the Alembic OArchive, otherwise HDF5 throws an exception.
Keep a reference to the stop, do_update and progress variables from
the export job in the exporter class, so the exporter can communicate
back to the main thread. This could be done nicer, but is ok for now.
Set the G.is_rendering flag and lock editor drawing during export.
This is a terribly hack which is necessary to prevent draw methods
accessing the same data that is written by the export job.
Disabled UNDO for the export operator, it doesn't make sense anyway.
Cache files are external outside the undo system and can not simply be
restored.