gst/gstbus.c: Fix order of members in GstBusSource structure - the first member must be the parent structure ie. GSou...

Original commit message from CVS:
* gst/gstbus.c: (gst_bus_set_main_context), (gst_bus_create_watch):
Fix order of members in GstBusSource structure - the first member
must be the parent structure ie. GSource. Should make bus sources
attached to non-default main contexts work in all cases now (ie.
primarily in cases where the callback has a non-NULL user data
argument). Fixes #562170.
* tests/check/gst/gstbus.c: (test_custom_main_context):
Add unit test for the above, based on code by
Justin Karneges <justin at affinix com>.