st-texture-cache: Add API to remove cache data

The current API assumes that image data loaded from files remains
valid during the life time of the shell. This assumption is mostly
valid for image files we provide ourselves (with the exception being
designers working on those files), but not necessarily for "external"
files - provide API to explicitly remove cached data associated with
a URI for those cases.

https://bugzilla.gnome.org/show_bug.cgi?id=679268