implement URI schemes

implement URI schemes

Original commit message from CVS:
implement URI schemes
Elements can now register as a source or sink for a protocol and applications can use gst_element_make_from_uri () to get an element that handles a given URI.
This patch provides:
- removal of old broken URI handling scheme.
- new URI handling using interfaces.
- updates for registry to save handled URIs.
- interface for URI handlers.
- implementation of that in filesrc and filesink for the file:// URI
- extension to pipeline parsing to allow specifying only a URI instead of element

Does not include:
- tests
- inclusion in docs build