gst/gstbin.c: Add default event/set_manager handlers. The set_manager handler takes care that the manager is distribu...

Original commit message from CVS:
* gst/gstbin.c: (gst_bin_class_init), (gst_bin_set_manager),
(gst_bin_send_event):
Add default event/set_manager handlers. The set_manager handler
takes care that the manager is distributed over kids that were
already in the bin before the manager was set. The event handler
is a utility virtual function that sends the event over all sinks,
so that gst_element_send_event (bin, event); has the expected
behaviour.
* gst/gstpad.c: (gst_pad_event_default):
Re-install default event handling for discontinuities, so that
seeking works without requiring hacks in applications or extra
code in sinks.
* gst/gstpipeline.c: (gst_pipeline_class_init),
(gst_pipeline_send_event):
Half hack, half utility: set a pipeline to PAUSED for seek events,
since that is the only way we can guarantee a/v sync. Means that
you can do gst_element_seek (pipeline, method, pos); on a pipeline
and it "just works".