Add context menus to some entries

Use ShellEntry.addContextMenu() to add context menus to most
existing entries, with the exception of:

 - the login dialog - it may act be used to enter either the
                      username (e.g. no password entry) or the
                      password, and copy/paste does not make sense
                      (nowhere to copy from, nowhere to paste to)
 - notifications    - while adding a context menu is useful here as
                      well, it will require changes to the tray's
                      focus grab handling, so leave those entries
                      out for now

https://bugzilla.gnome.org/show_bug.cgi?id=659275