Implement grabbing of the mouse pointer for X11

Contains new methods in the Window API that closely mirror the Xlib API. The
methods are left unimplemented for other platforms for now.

src/android/mod.rs

@@ -358,6 +358,10 @@ impl Window {
     pub fn set_cursor(&self, _: MouseCursor) {
     }
 
+    pub fn grab_cursor(&self) -> Result<(), String> { Ok(()) }
+
+    pub fn ungrab_cursor(&self) {}
+
     pub fn hidpi_factor(&self) -> f32 {
         1.0
     }