Add method to conveniently check the name of a custom event with gst_event_has_name().

Add method to conveniently check the name of a custom event with gst_event_has_name().

Original commit message from CVS:
* docs/gst/gstreamer-sections.txt:
* gst/gstevent.c: (gst_event_has_name):
* gst/gstevent.h:
* tests/check/gst/gstevent.c: (GST_START_TEST):
Add method to conveniently check the name of a custom event with
gst_event_has_name().
Reformat the event docs so that related methods are put together instead
of the default alphabetical sort.
Update unit test with new method.
API: GstEvent::gst_event_has_name()