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/Cargo.toml

@@ -14,6 +14,7 @@ features = ["winit"]
 iced_core = { version = "0.1.0-alpha", path = "../core" }
 stretch = "0.2"
 twox-hash = "1.5"
+raw-window-handle = "0.1"
 
 # Enable to obtain conversion traits
 winit = { version = "0.20.0-alpha3", optional = true }