Check if an object rthat has already been freed has been destroyed is not safe.
Add a helper function that uses weak reference to check that objects that are expected
to be destroyed when unrefing an object are actually destroyed.
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
