Make use of "g_volume_can_mount" to conditionally offer the option to mount

Once gvfs version 1.56.2 is shipped in all major distros, we can make use of the fixed g_volume_can_mount, which now should work reliable.

Like that, we can gray out the "mount" option when mounting is not possible for some reason, or when the volume is already mounted.

More info here: xfdesktop#343 (comment 97283)

And here: https://gitlab.gnome.org/GNOME/gvfs/-/issues/771

This change should be done in the same way for xfdesktop