Use shared thumbnails from remote locations

The shared thumbnails (originally in #262 (closed)) as implemented in !126 (merged) can not load remotely provided shared thumbnails.

AIU, this is (repeating #262 (comment 36098)) because

• xfce_create_shared_thumbnail_path creates a path (and not a URI) for any given input URI (which makes the location of the shared thumbnail unexpressible), and
• thunar_icon_factory_load_from_file taking a path (and not a URI) to feed into gdk_pixbuf_new_from_file (which, in what I perceive as somewhat inconsistent in the GTK ecosystem, doesn't have an equivalent URI function).

I've tentatively moved all involved values from paths to URIs, and taken a step through g_file_new_for_uri / g_file_read / gdk_pixbuf_new_from_stream, and lo and behold, thumbnails from network file systems. (I wholeheartedly recommend localtest:// for this exercise).

When this is resolved (with an MR coming up for thunar and utils), #634 can be tackled based on it.