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/workspacesView.js

@@ -17,9 +17,9 @@ const Tweener = imports.ui.tweener;
 const Workspace = imports.ui.workspace;
 const WorkspaceThumbnail = imports.ui.workspaceThumbnail;
 
-const WORKSPACE_SWITCH_TIME = 0.25;
+var WORKSPACE_SWITCH_TIME = 0.25;
 
-const AnimationType = {
+var AnimationType = {
     ZOOM: 0,
     FADE: 1
 };