gst/: Assign debug statements to relevant categories instead of the 'default' category so they don't get lost in debu...

Original commit message from CVS:
* gst/gstelement.c: (gst_element_message_full),
(gst_element_pads_activate):
* gst/gstobject.c: (gst_object_dispatch_properties_changed):
* gst/gstutils.c: (gst_pad_proxy_getcaps), (gst_pad_proxy_setcaps),
(gst_pad_add_data_probe_full), (gst_pad_add_event_probe_full),
(gst_pad_add_buffer_probe_full), (gst_pad_remove_data_probe),
(gst_pad_remove_event_probe), (gst_pad_remove_buffer_probe):
Assign debug statements to relevant categories instead of the 'default'
category so they don't get lost in debugging.