network: don't crash because a device doesn't have a description yet

Descriptions are only added after all devices are read (thanks
to the disambiguation in libnm-gtk), but we use them immediately
when we call _sync() in various points (such as checkConnection())

https://bugzilla.gnome.org/show_bug.cgi?id=706262
NMConnectionItem: fix typo

https://bugzilla.gnome.org/show_bug.cgi?id=706262
network: don't return null from NMConnectionDevice._getStatus()

StLabel complains set you set the text to NULL

https://bugzilla.gnome.org/show_bug.cgi?id=706262
network: use the VPN state to compute the icon for VPN items

We watch changes in the VPN state, not the active connection state,
so if we use the active connection state, we might miss an update
(because the VPN property is notified before the other one)

https://bugzilla.gnome.org/show_bug.cgi?id=706262
network: fix signal name

There is no state-changed signal on NMActiveConnection

https://bugzilla.gnome.org/show_bug.cgi?id=706262
network: allow disconnecting while activation is in progress

While connecting, the item should read "Turn Off", not "Connect".
To do so, change the meaning of isActive() to be really "not isOff()"

https://bugzilla.gnome.org/show_bug.cgi?id=706262