tests: integration: Add a list tests only option

Also add an empty filed in the GOptionEntry array avoiding segfaults
tests: integration: PNG file was renamed to png.png
tests: integration: Rework the way we handle seeking while fully paused

The idea is that we should first play until the time we reach the first
position, at that point we PAUSE the pipeline, then, afterward do the
seeks as asked.

If we get the position before the ASYNC DONE, just accept it.
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
formatter: Plug leaks in the can_save_to_uri method

https://bugzilla.gnome.org/show_bug.cgi?id=679941
ges-clip: when a child, is removed, disconnect from its notifies.