add a gst_element_set_state_async method that sets the state and starts a thread to make sure the state change comple...

Original commit message from CVS:
* check/gst/gstpipeline.c: (GST_START_TEST):
* docs/gst/gstreamer-sections.txt:
* gst/gstutils.c: (set_state_async_thread_func),
(gst_element_set_state_async):
* gst/gstutils.h:
add a gst_element_set_state_async method that
sets the state and starts a thread to make sure the state
change completes as best as it can