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/viewSelector.js

@@ -24,9 +24,9 @@ const EdgeDragAction = imports.ui.edgeDragAction;
 const IconGrid = imports.ui.iconGrid;
 
 const SHELL_KEYBINDINGS_SCHEMA = 'org.gnome.shell.keybindings';
-const PINCH_GESTURE_THRESHOLD = 0.7;
+var PINCH_GESTURE_THRESHOLD = 0.7;
 
-const ViewPage = {
+var ViewPage = {
     WINDOWS: 1,
     APPS: 2,
     SEARCH: 3