network: Don't use StButtons for items in selector

Their use blocks activation of the default button by keyboard, which
is important for accessibility. Use a Clutter.ClickAction instead,
which doesn't have this problem as it only considers mouse events.

https://bugzilla.gnome.org/show_bug.cgi?id=710144

data/theme/gnome-shell.css

@@ -306,16 +306,13 @@ StScrollBar StButton#vhandle:active {
     font-size: 12pt;
     border-bottom: 1px solid #666;
     padding: 12px;
-}
-
-.nm-dialog-item:checked {
-    background-color: #333;
-}
-
-.nm-dialog-item-box {
     spacing: 20px;
 }
 
+.nm-dialog-item:selected {
+    background-color: #333;
+}
+
 .nm-dialog-icons {
     spacing: .5em;
 }