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/status/accessibility.js

@@ -21,7 +21,7 @@ const KEY_MOUSE_KEYS_ENABLED        = 'mousekeys-enable';
 
 const APPLICATIONS_SCHEMA           = 'org.gnome.desktop.a11y.applications';
 
-const DPI_FACTOR_LARGE              = 1.25;
+var DPI_FACTOR_LARGE              = 1.25;
 
 const WM_SCHEMA                     = 'org.gnome.desktop.wm.preferences';
 const KEY_VISUAL_BELL               = 'visual-bell';