lookingGlass: fix to not overlap the keyboard

When the keyboard is configured, make lg shorter (if necessary) to
avoid overlapping it.

Also, make a few simplifications to lg's layout code. In particular,
move it into panelBox, to simplify its interactions with the panel.

https://bugzilla.gnome.org/show_bug.cgi?id=657986

js/ui/main.js

@@ -765,7 +765,6 @@ function popModal(actor, timestamp) {
 function createLookingGlass() {
     if (lookingGlass == null) {
         lookingGlass = new LookingGlass.LookingGlass();
-        lookingGlass.slaveTo(panel.actor);
     }
     return lookingGlass;
 }