st-texture-cache: Add texture-file-changed signal

For textures loaded from files, the cache might hide image changes
by keeping the data of a previous version around indefinitely. For
instance AccountsService will notify of avatar changes, but as new
image is copied over the old one, we will continue to use the old
image data.
Install a file monitor for each file resource we load and clear
the corresponding data from the cache on changes, emitting the
new StTextureCache::texture-file-changed signal.

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