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/userWidget.js

@@ -12,7 +12,7 @@ const St = imports.gi.St;
 
 const Params = imports.misc.params;
 
-const AVATAR_ICON_SIZE = 64;
+var AVATAR_ICON_SIZE = 64;
 
 // Adapted from gdm/gui/user-switch-applet/applet.c
 //