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.

examples/tour.rs

@@ -1,7 +1,7 @@
 use iced::{
-    button, slider, text::HorizontalAlignment, Align, Background, Button,
-    Checkbox, Color, Column, Element, Image, Justify, Length, Radio, Row,
-    Slider, Text, UserInterface,
+    button, slider, text::HorizontalAlignment, Align, Application, Background,
+    Button, Checkbox, Color, Column, Element, Image, Justify, Length, Radio,
+    Row, Slider, Text,
 };
 
 pub fn main() {
@@ -30,7 +30,7 @@ impl Tour {
     }
 }
 
-impl UserInterface for Tour {
+impl Application for Tour {
     type Message = Message;
 
     fn update(&mut self, event: Message) {