notification: Handle snap applications
notification: Handle snap applications

When a non-relocatable application is packaged as a snap, its resources
will be visible under `${SNAP}`. We can discover these resources by
probing paths that are given to us and trying the same path under
`${SNAP}` if the probe fails.

Snap packages have desktop files namespaced by the snap name. This name
is available inside the snap environment as `${SNAP_NAME}`. Also probe
desktop files and try this form of mangled name if the referenced
desktop file is not found.

GNOME/libnotify,GNOME/libnotify