Changes the interaction between the view controller and engine in both the C API and
the engine API, so that there's always an engine (as on other platforms) rather than
the engine APIs being specific to headless mode.
While adjusting the C API, this does a large cleanup:
- Renames all methods to follow a `FlutterDesktop` (prefix) + "class" name + method-style name.
E.g., `FlutterDestkopViewControllerCreate` rather than `FlutterDesktopCreateViewController`.
This makes it easier to see what functions operate on which conceptual "object" in the API.
- Reorders and groups them by the object they operate on.
Fixes https://github.com/flutter/flutter/issues/61966
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.
