PopupMenuManager was pretending that it knew nothing about the menu's
sourceActors, while also trying to handle keynav between them. This
was a big mess, and resulted in bugs in navigation between panel menus
and the Activities button, and it totally gets in the way when trying
to add keynav to the dash (whose menu sources are arranged vertically
rather than horizontally).
Fix this up by moving the panel-specific parts to PanelMenuButton
instead.
https://bugzilla.gnome.org/show_bug.cgi?id=641253
It is not referencing them when adding, and also it is connecting
to the "destroy" signal, emitted on dispose, so there is no risk
of storing finalized objects.
https://bugzilla.gnome.org/show_bug.cgi?id=634781
* Make sure all source files have a LGPL copyright header, and standardize
non-standard variations of the header to a common form.
* Check and update all copyright notices.
* Remove 'Written By:' lines. They are universally incomplete and
typically indicate only who started a particular file.
https://bugzilla.gnome.org/show_bug.cgi?id=634550
