libs/gst/base/: New GstDataQueue object for threadsafe queueing. Most useful for elements that need some queueing fun...

Original commit message from CVS:
* libs/gst/base/Makefile.am:
* libs/gst/base/gstdataqueue.c: (gst_data_queue_get_type),
(gst_data_queue_base_init), (gst_data_queue_class_init),
(gst_data_queue_init), (gst_data_queue_new),
(gst_data_queue_cleanup), (gst_data_queue_finalize),
(gst_data_queue_locked_flush), (gst_data_queue_locked_is_empty),
(gst_data_queue_locked_is_full), (gst_data_queue_flush),
(gst_data_queue_is_empty), (gst_data_queue_is_full),
(gst_data_queue_set_flushing), (gst_data_queue_push),
(gst_data_queue_pop), (gst_data_queue_drop_head),
(gst_data_queue_set_property), (gst_data_queue_get_property):
* libs/gst/base/gstdataqueue.h:
New GstDataQueue object for threadsafe queueing. Most useful for
elements that need some queueing functionnality.
* docs/libs/gstreamer-libs-docs.sgml:
* docs/libs/gstreamer-libs-sections.txt:
Insert documentation for GstDataQueue
* plugins/elements/Makefile.am:
* plugins/elements/gstelements.c:
* plugins/elements/gstmultiqueue.c: (gst_multi_queue_base_init),
(gst_multi_queue_class_init), (gst_multi_queue_init),
(gst_multi_queue_finalize), (gst_multi_queue_set_property),
(gst_multi_queue_get_property), (gst_multi_queue_request_new_pad),
(gst_multi_queue_release_pad), (gst_single_queue_push_one),
(gst_multi_queue_item_destroy), (gst_multi_queue_item_new),
(gst_multi_queue_loop), (gst_multi_queue_chain),
(gst_multi_queue_sink_activate_push), (gst_multi_queue_sink_event),
(gst_multi_queue_getcaps), (gst_multi_queue_bufferalloc),
(gst_multi_queue_src_activate_push), (gst_multi_queue_acceptcaps),
(gst_multi_queue_src_event), (gst_multi_queue_src_query),
(wake_up_next_non_linked), (compute_next_non_linked),
(single_queue_overrun_cb), (single_queue_underrun_cb),
(single_queue_check_full), (gst_single_queue_new):
* plugins/elements/gstmultiqueue.h:
New multiqueue element, using GstDataQueue. Used for queuing multiple
streams.
Closes #344639 and #347785