libs/gst/base/gstbasesrc.*: Add property to make the basesrc timestamp buffers based on the current running time.

Original commit message from CVS:
* libs/gst/base/gstbasesrc.c: (gst_base_src_class_init),
(gst_base_src_init), (gst_base_src_set_do_timestamp),
(gst_base_src_get_do_timestamp), (gst_base_src_set_property),
(gst_base_src_get_property), (gst_base_src_do_sync):
* libs/gst/base/gstbasesrc.h:
Add property to make the basesrc timestamp buffers based on the current
running time.
API: GstBaseSrc::do-timestamp
API: gst_base_src_set_do_timestamp()
API: gst_base_src_get_do_timestamp()