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/main.js

@@ -552,7 +552,7 @@ function activateWindow(window, time, workspaceNum) {
 
 // TODO - replace this timeout with some system to guess when the user might
 // be e.g. just reading the screen and not likely to interact.
-const DEFERRED_TIMEOUT_SECONDS = 20;
+var DEFERRED_TIMEOUT_SECONDS = 20;
 var _deferredWorkData = {};
 // Work scheduled for some point in the future
 var _deferredWorkQueue = [];