Bug 597864 – Need to separate "Safely Remove Drive" from "Eject"

Some systems use internal USB devices. Automatically detaching such
drives when calling gdu_drive_eject() (triggered by e.g. the eject
button in the Nautilus sidebar) as we do right now leads to loss of
the drive until reboot (since the device is internal). The only viable
option right now is to separate "Safely Remove Drive" and "Eject".

If we gain infrastructure to let us know with 100% accuracy that a
given receptacle is external we can go back to the previous behavior.

See http://bugs.freedesktop.org/show_bug.cgi?id=24343 for details.