- Added new `renderer::Windowed` trait. This shoud allow users to easily
try different renderers by simply changing one line.
- Renamed `UserInterface` traits to `Application`, as the `run` method
takes total control of the current thread.
- Moved `MouseCursor` back to `iced_native`. The new
`renderer::Windowed` trait returns one on `draw`.
- Split `iced_native` renderer in multiple modules, for consistency.