ScreenShield: show the unlock dialog on the primary monitor when using the keyboard

Make ModalDialog.open() accept an optional onPrimary argument, and
pass it when the dialog is activated using ESC or Return.

https://bugzilla.gnome.org/show_bug.cgi?id=685855

js/ui/layout.js

@@ -43,6 +43,8 @@ const MonitorConstraint = new Lang.Class({
     },
 
     set primary(v) {
+        if (v)
+            this._index = -1;
         this._primary = v;
         if (this.actor)
             this.actor.queue_relayout();
@@ -54,6 +56,7 @@ const MonitorConstraint = new Lang.Class({
     },
 
     set index(v) {
+        this._primary = false;
         this._index = v;
         if (this.actor)
             this.actor.queue_relayout();