gst/gstbus.*: Implement a real GSource and use g_main_context_wakeup() to signal new messages instead of the socketpair.

Original commit message from CVS:
* gst/gstbus.c: (gst_bus_init), (gst_bus_dispose), (gst_bus_post),
(gst_bus_pop), (gst_bus_source_prepare), (gst_bus_source_check),
(gst_bus_source_dispatch), (gst_bus_source_finalize),
(gst_bus_create_watch), (gst_bus_add_watch_full):
* gst/gstbus.h:
Implement a real GSource and use g_main_context_wakeup() to
signal new messages instead of the socketpair.