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.

native/src/lib.rs

@@ -206,6 +206,7 @@ mod element;
 mod event;
 mod hasher;
 mod layout;
+mod mouse_cursor;
 mod node;
 mod style;
 mod user_interface;
@@ -223,6 +224,7 @@ pub use element::Element;
 pub use event::Event;
 pub use hasher::Hasher;
 pub use layout::Layout;
+pub use mouse_cursor::MouseCursor;
 pub use node::Node;
 pub use renderer::Renderer;
 pub use style::Style;