gst/gstelement.c: Expose default element send_event and query handling as vmethods that subclasses can chain up to.

Original commit message from CVS:
* gst/gstelement.c: (gst_element_class_init),
(gst_element_default_send_event), (gst_element_send_event),
(gst_element_default_query), (gst_element_query):
Expose default element send_event and query handling as vmethods that
subclasses can chain up to.