This cascaded into a lot more work than expected. However, in general,
if one clones a `WidgetInstance` and shares it between two windows, it
should now work. Widget authors must ensure that when they cache
information, they do so with either a `WidgetCacheKey` or use a
`WindowLocal<T>` if per-window state is desired.
This is demonstrated in the debug-window example, where the counter of
open windows is next to a clone of the same button from the main window
that opens a new window.
