icon-theme: support loading symbolic GFileIcons from generic URIs

icon-theme: support loading symbolic GFileIcons from generic URIs

Right now we support loading and recoloring symbolic GFileIcons, but
only if the underlying GFile has a local path. This breaks when the
GFileIcon is loaded from a GResource, which is a reasonable option for an
application that wants to ship a custom symbolic icon.

This patch changes GtkIconInfo to store a GFile together with the file
path, and changes the symbolic icon lookup code to use the GFile URI,
which transparently makes the code work also for GResources.

https://bugzilla.gnome.org/show_bug.cgi?id=687059