applets/budgie-menu: Greatly improve keyboard navigation

It makes no sense for the categories section to have keyboard focus,
as it will merely break tab cycling within the menu. Now the user
may simply press down on their keyboard to scroll through entries,
or indeed type first, then go "down" or tab across (optional) to
select the appropriate option, all without having to use the mouse
whatsoever.

We also now support invocation of an item by using the enter key
upon keyboard focus, which makes the mouse unnecessary in these
instances.