gst/: make sure some essential types used by events are registered as part of gst_init()

gst/: make sure some essential types used by events are registered as part of gst_init()

Original commit message from CVS:
* gst/gstevent.c: (_gst_event_initialize):
* gst/gstformat.c: (_gst_format_initialize):
make sure some essential types used by events are registered
as part of gst_init()
* gst/gstvalue.c: (gst_value_serialize_flags):
if no flags are set, serialize them to a value that represents NONE
so that deserializing them works
* tests/check/gst/gstvalue.c: (GST_START_TEST), (gst_value_suite):
add tests for serialization and deserialization of flags