ges-formatter: Check if directory of URI is writeable

https://bugzilla.gnome.org/show_bug.cgi?id=679941
formatter: Remove the unsed can_save_uri vmethod

This virtual method does not make much sense right now, we might need it
again later, but most probably with a sensibly different API so removing
it for now.
pipeline: Remove the dynamic lock

We actually do not need it has everywhere where we would need it we are
already locked against the timeline.dyn_lock, we need to make sure it is
always the case in the future.

The hierarchy of the mutex was wrong and could possibly lead to
deadlocks