AirplaneMode: don't show a "Turn off" item if HW enabled

If airplane mode is enabled with an HW switch, we can't turn it
off from the menu, so make the menu item insensitive and
replace it with a helpful tip.

https://bugzilla.gnome.org/show_bug.cgi?id=709685
rfkill: split out the dbus handling parts from the indicator

status.network wants to watch for airplane mode too, we can
share the code with a manager singleton.

https://bugzilla.gnome.org/show_bug.cgi?id=709128
NMWirelessDialog: fix removing access points

We must destroy the actor, not the item, because the latter doesn't
have a destroy() method.

https://bugzilla.gnome.org/show_bug.cgi?id=709128
NMWirelessDialog: request a scan when there are no visible access points

Ideally, we should keep scanning while the dialog is open, and
stop (or reduce the frequency) when it's closed. Forcing a new
scan when the dialog is opened empty is a close approximation
and increases the probability that the user will find the AP
he needs.

https://bugzilla.gnome.org/show_bug.cgi?id=709128
NMWirelessDialog: show an informative message when airplane mode is on

Rather than just showing "No networks", inform the user if airplane
mode is on or if wifi is off, and allow the user to change that
from the dialog (if possible).

https://bugzilla.gnome.org/show_bug.cgi?id=709128
NetworkMenu: use the carrier state to the decide if wired should be shown

The design says "when a network cable is connected", not
"When a network cable is connected and the link is up and we
have an IP etc. etc." (which is what ACTIVATED would imply).

https://bugzilla.gnome.org/show_bug.cgi?id=723570