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/renderer/windowed.rs

@@ -0,0 +1,17 @@
+use crate::MouseCursor;
+
+use raw_window_handle::HasRawWindowHandle;
+
+pub trait Windowed: super::Renderer {
+    type Target;
+
+    fn new<W: HasRawWindowHandle>(window: &W) -> Self;
+
+    fn target(&self, width: u16, height: u16) -> Self::Target;
+
+    fn draw(
+        &mut self,
+        target: &mut Self::Target,
+        primitive: &Self::Primitive,
+    ) -> MouseCursor;
+}