gThumb should check and, if it's the case, mount the device before trying to
import photos from a PTP camera.

This causes a bug in the import function, which doesn't work in non-GNOME
desktop environments (i.e. Xfce). GNOME, in fact, automounts cameras as they're
plugged in, and gThumb could then do the import.

For Xfce, a workaround is to add "gvfs-mount %d; " to the Exec line in
gthumb-import.desktop.