gnlcomposition: add eos_thread_function to replace eos_main_thread.

	+ This is a WIP commit.
composition: Cleanup and fix commit introducing our own thread

Update the pipeline from the streaming thread before emiting EOS
gnlcomposition: When removing a ghostpad, remove the probe_handlers associated to its target.
gnlcomposition: remove unused pending_idle private member
composition: Rename "eos thread" to "update pipeline thread"
composition: Add comments about updating the pipeline from the streaming thread