Rely on the GstElement to properly handle their seqnums

Actually it is not exactly thread safe to tweak them ourself at the GNL
level.
composition: set next_eos_seqnum when we get seeked too.
composition: use g_main_context_set_dispatches_per_iteration ()

Let's hope this gets merged ...
composition: initialize if it wasn't already, damn you!
object: make the check for self commit work in a crappy way.