Don't declare global variables

Fix a typo in panel.js, and ensure that all variables used in
functions are scoped to the block (using let), to avoid polluting
the global namespace.

https://bugzilla.gnome.org/show_bug.cgi?id=643210

js/ui/environment.js

@@ -17,7 +17,7 @@ function _patchContainerClass(containerClass) {
     // This one is a straightforward mapping of the C method
     containerClass.prototype.child_set = function(actor, props) {
         let meta = this.get_child_meta(actor);
-        for (prop in props)
+        for (let prop in props)
             meta[prop] = props[prop];
     };