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
33b8537bf5