gst/base/: Make basesrc negotiate.

gst/base/: Make basesrc negotiate.

Original commit message from CVS:
* gst/base/README:
* gst/base/gstbasesink.c: (gst_base_sink_preroll_queue_empty),
(gst_base_sink_handle_object), (gst_base_sink_loop),
(gst_base_sink_change_state):
* gst/base/gstbasesink.h:
* gst/base/gstbasesrc.c: (gst_base_src_class_init),
(gst_base_src_init), (gst_base_src_setcaps),
(gst_base_src_getcaps), (gst_base_src_loop),
(gst_base_src_default_negotiate), (gst_base_src_negotiate),
(gst_base_src_start), (gst_base_src_change_state):
* gst/base/gstbasesrc.h:
Make basesrc negotiate.
Handle the case where preroll fails in basesink.
Update README.