Cut down on boilerplate by using the (no longer that) new helper
macros. We don't care about breaking ABI in private libraries, so
use G_DECLARE_FINAL_TYPE even where the class struct used to be
exposed in the header, except for types we inherit from ourselves
(obviously) or where the class exposes any vfuncs (where changes
could affect inheritance in extensions).
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
* 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
