Widgets should have opportunity to decline losing focus

Nobody has claimed this yet.

Currently, if the user tabs away from a field or another widget tries to take focus, there is no clear way for the currently focused widget to override the focus loss, without doing hacky things like sending itself a `Command` and taking focus back immediately afterwards. (which is what the textbox does).

This feels bad; there are lots of cases where you want a widget to be able to decline a focus change, such as when its current contents are invalid.

I think this should be fairly easy to manage by adding an additional lifecycle event; I think the pattern I would go with here is the `Cell` trick we use in some of the debug events, where we pass a `Cell` along with the event and then the widget can set this flag if it wants to retain focus.

Start by tracing the focus-change lifecycle and reviewing the existing debug events that use the Cell pattern. Define the lifecycle event's veto behavior for the currently focused widget, then verify that widgets can retain focus without sending a Command to reclaim it.

Written by the indexing model from the issue text.