Better support for dynamically built menus

[bugadani]

Currently, dynamic lists of menu items are only possible in a very restricted way: it is not possible to arbitrarily mix and match navigation and selection items.

Unfortunately, we can't make the menu items object-safe because:

• e-g's drawables aren't object safe
• trait objects can't contain generic methods (like draw).

What we can do instead is to wrap menu items in an enum. We only need this to support "polymorphism" in add_items, and there are two key questions to consider:

• ergonomics
• support user-defined menu items (i.e. easily define a wrapper enum around N specific menu item impls)

[BroderickCarlin]

I'm not entirely sure if this is within the same vein of this issue, but is it being considered changing embedded-menu to be immediate mode rather than the current retained mode patterns? This would enable a developer to more easily create dynamic menus without requiring embedded-menu to develop a way to handle complex and mutable menu topology.

[bugadani]

I've never considered immediate mode. I also don't have neither the knowledge nor enough time to make such a fundamental change to embedded-menu, unfortunately.