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
Add addContextMenu() to support context menus on right-click/long-press.
Depending on the parameters passed, the context menu only contains
"Copy"/"Paste" actions or an additional "Show/Hide Text" toggle action
for password entries.
https://bugzilla.gnome.org/show_bug.cgi?id=659275