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/screenshot.js

@@ -338,7 +338,7 @@ var SelectArea = new Lang.Class({
 });
 Signals.addSignalMethods(SelectArea.prototype);
 
-const FLASHSPOT_ANIMATION_OUT_TIME = 0.5; // seconds
+var FLASHSPOT_ANIMATION_OUT_TIME = 0.5; // seconds
 
 var Flashspot = new Lang.Class({
     Name: 'Flashspot',