gst/gstbus.c: Make GstBusSource work with non-default main contexts (#562170).

gst/gstbus.c: Make GstBusSource work with non-default main contexts (#562170).

Original commit message from CVS:
* gst/gstbus.c: (gst_bus_dispose), (gst_bus_get_property),
(gst_bus_wakeup_main_context), (gst_bus_set_main_context),
(gst_bus_post), (gst_bus_source_prepare), (gst_bus_source_finalize),
(gst_bus_create_watch):
Make GstBusSource work with non-default main contexts (#562170).
* tests/check/gst/gstbus.c: (message_func_eos), (message_func_app),
(test_watch), (test_watch_with_custom_context), (gst_bus_suite):
Add test case for GstBusSource with a non-default main context.
* tests/check/libs/.cvsignore:
Ignore more.