ui: Improve handling being headless

Don't assume there will always be a primary (logical) monitor, or any
(logical) monitor at all. This includes not allocating / layouting /
styling correctly when being headless.

The initial background loading will also be delayed until there are any
(logical) monitors connected.

https://bugzilla.gnome.org/show_bug.cgi?id=730551

js/ui/overview.js

@@ -392,6 +392,9 @@ var Overview = new Lang.Class({
         // when it is next shown.
         this.hide();
 
+        if (!Main.layoutManager.primaryMonitor)
+            return;
+
         let workArea = Main.layoutManager.getWorkAreaForMonitor(Main.layoutManager.primaryIndex);
 
         this._coverPane.set_position(0, workArea.y);