grab_cursor and hide_cursor (#571)

* Windows: Use new cursor state API

* X11: Use new cursor state API

* macOS: Use new cursor state API

* Android+iOS: Stubbed new cursor state API

* Emscripten: Use new cursor state API

* Prevent multiple inc/dec of display count on Windows

* Fixed missing imports (no idea where those went)

* Remove NoneCursor

* Improved documentation

* Fix Emscripten build

* Windows: Re-grab before and after fullscreen

examples/cursor_grab.rs

@@ -0,0 +1,38 @@
+extern crate winit;
+
+fn main() {
+    let mut events_loop = winit::EventsLoop::new();
+
+    let window = winit::WindowBuilder::new()
+        .with_title("Super Cursor Grab'n'Hide Simulator 9000")
+        .build(&events_loop)
+        .unwrap();
+
+    events_loop.run_forever(|event| {
+        if let winit::Event::WindowEvent { event, .. } = event {
+            use winit::WindowEvent::*;
+            match event {
+                CloseRequested => return winit::ControlFlow::Break,
+                KeyboardInput {
+                    input: winit::KeyboardInput {
+                        state: winit::ElementState::Released,
+                        virtual_keycode: Some(key),
+                        modifiers,
+                        ..
+                    },
+                    ..
+                } => {
+                    use winit::VirtualKeyCode::*;
+                    match key {
+                        Escape => return winit::ControlFlow::Break,
+                        G => window.grab_cursor(!modifiers.shift).unwrap(),
+                        H => window.hide_cursor(!modifiers.shift),
+                        _ => (),
+                    }
+                }
+                _ => (),
+            }
+        }
+        winit::ControlFlow::Continue
+    });
+}