Original commit message from CVS:
Added mthodes to request an element to create pads: gst_element_request_pad*
This can be used to construct a tee and a muxer/mixer/aggregator element.
Moved the tee element to elements/ because it can now be handled with the
new pad request features.
The padfactory also has some changes: a pad can now be of presence REQUEST,
which means that the pad can be requested from this plugin (doh).
