notificationDaemon: support file:// or icon theme names for image-path

notificationDaemon: support file:// or icon theme names for image-path

While this sounds counter-intuitive, the image-path hint value might also
be used with URIs or icon names.

As per freedesktop standard:
  The "app_icon" parameter and "image-path" hint should be either an URI
  (file:// is the only URI schema supported right now) or a name in a
  freedesktop.org-compliant icon theme (not a GTK+ stock ID).

Thus the image-path hint should also be parsed as it happens for the
app_icon.

Reuse same logic, by falling back on _iconForNotificationData with the
hint value.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/285