Provide eject button on side pane for volumes

Add an eject icon to the shortcuts sidebar for volumes which are ejectable or
mounts which are unmountable by the user. This makes it much more obvious and
easy how to unmount/eject removable storage again.

http://bugzilla.xfce.org/show_bug.cgi?id=3658



Signed-off-by: Jannis Pohlmann <jannis@xfce.org>