Panel: clean up button construction code

We already could build the right part of the panel declaratively according
to the session mode. Extend that to handle the left and center parts.
Also, move the mapping from the roles to the classes in panel.js, as it shared
by all modes.

https://bugzilla.gnome.org/show_bug.cgi?id=682546

js/ui/main.js

@@ -222,8 +222,7 @@ function start() {
 
     sessionMode.createSession();
 
-    panel.startStatusArea();
-
+    panel.init();
     layoutManager.init();
     keyboard.init();
     overview.init();