ges: Port to the new commit based API in GNL

ges: Port to the new commit based API in GNL

The GNL API changed to go from a model where user could
enable/disable updates in the composition, which leaded to races
in many places, to a model where any positioning change in the
composition is not directly done but 'cached' and then the user
has to commit those changes so they become effective in the media
processing stack.

The new API in GES is pretty similare and is basically copy
pasting this new design.

We still need to see if in some context it would make sense to add
a mode where we would commit any changes ourself at the end of our
operation for basic use cases.

Removed APIs:
  ges_timeline_enable_update
  ges_timeline_is_updating
  ges_track_enable_update
  ges_track_is_updating

New APIs:
  ges_track_commit
  ges_timeline_commit