popupMenu, panelMenu: split up panel and non-panel keynav

popupMenu, panelMenu: split up panel and non-panel keynav

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