Take a `Keymap` when setting app menus

For a brief period on this branch, we were taking a `DispatchTree`. Doing so
resulted in more accurate key bindings but it meant that we would have had to
recompute the app menus every time the key context changed.

We decided to err on the side of keeping things simple and work in the same
way they worked back in zed1.

Co-Authored-By: Marshall <marshall@zed.dev>