Add function that (unlike the GLib equivalent) also accepts paths that
aren't absolute and will clean up relative markers such as ./ and ../
before forming a URI.

Fixes warnings with e.g. filesrc location=foo ! typefind caused by the
recent switch to g_filename_to_uri(), but also actually creates valid
URIs for the first time.

Windows code paths could need some more work, e.g. we don't clean up
the relative markers there for now (because path could have \ and /
as separators).

API: gst_filename_to_uri()