Add new API: gst_pipeline_set_auto_flush_bus() and gst_pipeline_get_auto_flush_bus() to disable automatic flushing of...

Original commit message from CVS:
* docs/gst/gstreamer-sections.txt:
* gst/gstpipeline.c: (gst_pipeline_class_init),
(gst_pipeline_init), (gst_pipeline_set_property),
(gst_pipeline_get_property), (gst_pipeline_change_state),
(gst_pipeline_set_auto_flush_bus),
(gst_pipeline_get_auto_flush_bus):
* gst/gstpipeline.h:
Add new API: gst_pipeline_set_auto_flush_bus() and
gst_pipeline_get_auto_flush_bus() to disable automatic
flushing of the pipeline's GstBus when going from READY
to NULL state (#332045).