gst/gstindex.*: Add new function with option to dispose of user_data in resolver.

gst/gstindex.*: Add new function with option to dispose of user_data in resolver.

Original commit message from CVS:
Patch by: Siavash Safi <siavash dot safi at gmail dot com>
* gst/gstindex.c: (gst_index_finalize), (gst_index_set_resolver),
(gst_index_set_resolver_full):
* gst/gstindex.h:
Add new function with option to dispose of user_data in resolver.
Actually call the dispose function when finalizing the object and not
just when changing the resolver/filter.
API: GstIndex::gst_index_set_resolver_full()
* docs/gst/gstreamer-sections.txt:
Add new function to docs. Fixes #515469.