modal widgets

Nobody has claimed this yet.

Doing modal dialogs correctly involves a lot of platform coordination, and so I would like as a stopgap to add support for modal widgets; these will be usable for things like a drop-down combo box or a setting panel, allowing us to implement working versions of these things much more easily than if we try and hook platform windows for all of them.

There are some API questions: for instance, a combo box needs to be presented with its current selection aligned at a particular location, and also needs to be maximally visible since we won't be able to draw outside the bounds of the containing window.

The _general_ architecture, though, should be pretty simple: Only one modal will be allowed at a time, it will exist at the window level, and it will communicate back to the main widget tree by sending `Command`s when an item is selected. We could also add life cycle events for showing and hiding a modal.

I'm not sure where exactly this is on the roadmap, but it's probably something we should do soon. I know that @xStrom has been working on some related stuff, and so if they're interested in exploring this they would be very welcome; otherwise (unless anyone else is interested) I'll take it up sometime after the life cycle work.

Start by reviewing the existing window-level and main widget-tree architecture, especially how Commands and lifecycle events are represented. Define the modal widget API and its positioning and visibility rules, then implement single-modal coordination and selection communication once those API questions are resolved.

Written by the indexing model from the issue text.