Port PopupMenu to new Lang.Class framework

Port PopupMenu to new Lang.Class framework

The Lang module in gjs has recently gained a small yet powerful
Class framework, that should help improve the readability of code
when using complex inheritance.
This commit starts porting shell code, by rewriting all classes in
popupMenu.js (and all derived classes) to Lang.Class.

https://bugzilla.gnome.org/show_bug.cgi?id=664436