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/window.rs

@@ -414,6 +414,22 @@ impl Window {
     pub fn set_cursor_position(&self, x: i32, y: i32) -> Result<(), ()> {
         self.window.set_cursor_position(x, y)
     }
+
+    /// Grabs the mouse cursor. The cursor's motion will be confined to this
+    /// window and the window has exclusive access to further events regarding
+    /// the cursor.
+    /// Fails if it is not possible to grab the window for some reason, e.g.
+    /// when another window has already done so.
+    /// Has no effect on Android.
+    pub fn grab_cursor(&self) -> Result<(), String> {
+        self.window.grab_cursor()
+    }
+
+    /// Release a previously grabbed mouse cursor.
+    pub fn ungrab_cursor(&self) {
+        self.window.ungrab_cursor();
+    }
+
 }
 
 impl gl_common::GlFunctionsSource for Window {