Originally font change notification was handled by forwarding
WM_FONTCHANGE to the Flutter HWND, to avoid adding new API surface, but
that's not a good solution in a multi-window scenario, and it would
require a completely different solution for UWP. It also requires
non-obvious plumbing in the runner.
This replaces that with an explicit API, so that there's a clean and
obvious way for the runner to trigger this event.
