libs/gst/base/gstbasesink.*: Add async property to instruct the sink never to inform the parent about

Original commit message from CVS:
* libs/gst/base/gstbasesink.c: (gst_base_sink_class_init),
(gst_base_sink_init), (gst_base_sink_set_sync),
(gst_base_sink_get_sync), (gst_base_sink_set_max_lateness),
(gst_base_sink_get_max_lateness), (gst_base_sink_set_qos_enabled),
(gst_base_sink_is_qos_enabled), (gst_base_sink_set_async_enabled),
(gst_base_sink_is_async_enabled), (gst_base_sink_set_property),
(gst_base_sink_get_property), (gst_base_sink_change_state):
* libs/gst/base/gstbasesink.h:
Add async property to instruct the sink never to inform the parent about
ASYNC state changes, update docs.
Check argument with g_return_* for the public functions.
API: GstBaseSink::async property
API: gst_base_sink_set_async_enabled()
API: gst_base_sink_is_async_enabled()