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
e297b7d731