Define externally accessible contants with 'var' instead of 'const'

Just as we did with classes, define other constants that are (or
may be) used from other modules with 'var' to cut down on warnings.

https://bugzilla.gnome.org/show_bug.cgi?id=785084

js/ui/edgeDragAction.js

@@ -8,8 +8,8 @@ const St = imports.gi.St;
 
 const Main = imports.ui.main;
 
-const EDGE_THRESHOLD = 20;
-const DRAG_DISTANCE = 80;
+var EDGE_THRESHOLD = 20;
+var DRAG_DISTANCE = 80;
 
 var EdgeDragAction = new Lang.Class({
     Name: 'EdgeDragAction',