Add mouse cursor support

Add a new api, window.set_cursor, for setting the cursor. The enum MouseCursor lists the possible cursors.

Only X11 is implemented. On OSX, Android, & Win32 the window.set_cursor function
either does nothing or calls the "unimplemented!" macro.

src/android/mod.rs

@@ -280,6 +280,9 @@ impl Window {
 
     pub fn set_window_resize_callback(&mut self, _: Option<fn(uint, uint)>) {
     }
+
+    pub fn set_cursor(&self, cursor: MouseCursor) {
+    }
 }
 
 unsafe impl Send for Window {}