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.
Refactors the Windows embedding internals to make an engine object that
owns things associated with the engine rather than the view, and updates
the API surface to allow using the engine directly.
This is an incremental step toward both a cleaner, non-struct-based
internal structure and a finalized API surface.
