gst/elements/: Use gst_pad_proxy_getcaps().

gst/elements/: Use gst_pad_proxy_getcaps().

Original commit message from CVS:
* gst/elements/gstaggregator.c:
* gst/elements/gsttee.c:
Use gst_pad_proxy_getcaps().
* gst/gstpad.c:
* gst/gstpad.h:
Add gst_pad_proxy_getcaps(), which filter elements can use
as a generic getcaps implementation.
Fix gst_pad_get_allowed_caps().  It just wasn't doing what
was advertised.