tracker: Add test_media_from_uri

Given an URL, tests if Tracker source can create a media out of it.

https://bugzilla.gnome.org/show_bug.cgi?id=678519
tracker: Add media_from_uri operation

Given an URL, re-constructs the GrlMedia corresponding to that URL.

https://bugzilla.gnome.org/show_bug.cgi?id=678519