network: Rework the VPN rewrite to work for modem and bluetooth as well

Replace NMNetworkMenuItem with NMConnectionItem, based on
NMVPNConnectionItem, and replace NMDevice with NMConnectionSection
and NMConnectionDevice.

Since this rips apart NMDevice, and since wi-fi should not be
connection-based, we'll temporarily remove NMDeviceWireless. We'll
add it back in a later commit, along with the new Wi-Fi dialog.

https://bugzilla.gnome.org/show_bug.cgi?id=704670

data/theme/gnome-shell.css

@@ -262,10 +262,6 @@ StScrollBar StButton#vhandle:active {
     background-size: contain;
 }
 
-.nm-menu-item-icons {
-    spacing: .5em;
-}
-
 /* Buttons */
 
 .candidate-page-button,