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/mouse_cursor.rs

@@ -0,0 +1,35 @@
+/// The state of the mouse cursor.
+#[derive(Debug, Eq, PartialEq, Clone, Copy)]
+pub enum MouseCursor {
+    /// The cursor is out of the bounds of the user interface.
+    OutOfBounds,
+
+    /// The cursor is over a non-interactive widget.
+    Idle,
+
+    /// The cursor is over a clickable widget.
+    Pointer,
+
+    /// The cursor is over a busy widget.
+    Working,
+
+    /// The cursor is over a grabbable widget.
+    Grab,
+
+    /// The cursor is grabbing a widget.
+    Grabbing,
+}
+
+#[cfg(feature = "winit")]
+impl From<MouseCursor> for winit::window::CursorIcon {
+    fn from(mouse_cursor: MouseCursor) -> winit::window::CursorIcon {
+        match mouse_cursor {
+            MouseCursor::OutOfBounds => winit::window::CursorIcon::Default,
+            MouseCursor::Idle => winit::window::CursorIcon::Default,
+            MouseCursor::Pointer => winit::window::CursorIcon::Hand,
+            MouseCursor::Working => winit::window::CursorIcon::Progress,
+            MouseCursor::Grab => winit::window::CursorIcon::Grab,
+            MouseCursor::Grabbing => winit::window::CursorIcon::Grabbing,
+        }
+    }
+}