Removed plugable schedulers.

Removed plugable schedulers.

Original commit message from CVS:
Removed plugable schedulers.
Removed Scheduler/Manager from elements.
Removed gsttypes.h, rearranged includes.
Removed dependency pad<->element, element<>pipeline, and
various others,  fix includes.
implement gst_pad_get_parent() with gst_object_get_parent()
Make GstTask sefcontained.
Fix _get_state() on GstBin, it did not return ASYNC with a 0
timeout.
Fix endless loop in iterator_fold_with_resync.