gst/schedulers/gstoptimalscheduler.c: Fixes to maintain internal consistency of the...

gst/schedulers/gstoptimalscheduler.c: Fixes to maintain internal consistency of the scheduler data structures.

Original commit message from CVS:
* gst/schedulers/gstoptimalscheduler.c: (add_to_chain),
(remove_from_chain), (chain_group_set_enabled), (add_to_group),
(merge_groups), (setup_group_scheduler),
(group_inc_links_for_element), (gst_opt_scheduler_iterate),
(gst_opt_scheduler_show):
Fixes to maintain internal consistency of the scheduler data
structures.
- adding an enabled group to a chain should increment the
number of enabled elements in that chain.
- removing an enabled group from a chain could disable the
chain.
- removing a disabled group from a chain could enable the
chain.
- add g_assert when internal inconsistency is detected.
- adding an element to a group could increase the number of
links this group has with other groups.
- merging two groups also merges the chains.
- also show group links in the _show method.

Please report regressions ASAP, so I can fix them.