network: Fix possible crash when wireless-enabled changes

The device we're handling might have disappeared already, but we're
listening on the NMClient object, which is still alive.

https://bugzilla.gnome.org/show_bug.cgi?id=735932
network: Fix more potential crashers for WWAN

And virtual devices. Again, listening on the long-lasting client
object but expecting the user_data (the object) to be around when
it might have been destroyed already.

https://bugzilla.gnome.org/show_bug.cgi?id=735932
datetime: Don't hard-code font size

Use a percentage instead.

https://bugzilla.gnome.org/show_bug.cgi?id=735973