string freeze break for the network panel

[Matthias Clasen <matthias clasen gmail com>]

Here are two more patches for the control-center network panel:

https://bugzilla.gnome.org/show_bug.cgi?id=645790
https://bugzilla.gnome.org/show_bug.cgi?id=645772

The first one fixes problems with incorrect refreshing of the UI when
activating VPN connections. We were not tracking the state of the
connection, so the UI gets stuck on an intermediate state like
'Authentication required' and you have to switch tabs an back to get
'Connected'.

The second one is large due to the fact that it changes the layout in
the ui file from boxes + sizegroups to grids. The effect is subtle but
important: things no longer jiggle around as you switch between
devices, and the headers are perfectly aligned with the content. This
gives the UI a much more 'solid' feeling.