Move winit logic from iced to iced_winit

- 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.

wgpu/src/lib.rs

@@ -1,4 +1,3 @@
-mod mouse_cursor;
 mod primitive;
 mod quad;
 mod renderer;
@@ -7,6 +6,5 @@ mod transformation;
 pub(crate) use quad::Quad;
 pub(crate) use transformation::Transformation;
 
-pub use mouse_cursor::MouseCursor;
 pub use primitive::Primitive;
 pub use renderer::{Renderer, Target};