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.

examples/cursor.rs

@@ -0,0 +1,49 @@
+#[cfg(target_os = "android")]
+#[macro_use]
+extern crate android_glue;
+
+extern crate glutin;
+
+use glutin::{Event, ElementState, MouseCursor};
+
+mod support;
+
+#[cfg(target_os = "android")]
+android_start!(main);
+
+#[cfg(not(feature = "window"))]
+fn main() { println!("This example requires glutin to be compiled with the `window` feature"); }
+
+#[cfg(feature = "window")]
+fn main() {
+    
+    let mut window = glutin::Window::new().unwrap();
+    window.set_title("A fantastic window!");   
+    unsafe { window.make_current() };
+
+    let context = support::load(&window);
+    let cursors = [MouseCursor::Default, MouseCursor::Crosshair, MouseCursor::Hand, MouseCursor::Arrow, MouseCursor::Move, MouseCursor::Text, MouseCursor::Wait, MouseCursor::Help, MouseCursor::Progress, MouseCursor::NotAllowed, MouseCursor::ContextMenu, MouseCursor::NoneCursor, MouseCursor::Cell, MouseCursor::VerticalText, MouseCursor::Alias, MouseCursor::Copy, MouseCursor::NoDrop, MouseCursor::Grab, MouseCursor::Grabbing, MouseCursor::AllScroll, MouseCursor::ZoomIn, MouseCursor::ZoomOut, MouseCursor::EResize, MouseCursor::NResize, MouseCursor::NeResize, MouseCursor::NwResize, MouseCursor::SResize, MouseCursor::SeResize, MouseCursor::SwResize, MouseCursor::WResize, MouseCursor::EwResize, MouseCursor::NsResize, MouseCursor::NeswResize, MouseCursor::NwseResize, MouseCursor::ColResize, MouseCursor::RowResize];
+    let mut cursor_idx = 0;
+    
+    while !window.is_closed() {
+        context.draw_frame((0.0, 1.0, 0.0, 1.0));
+        window.swap_buffers();
+
+        for event in window.wait_events() {
+            match event {
+                Event::KeyboardInput(ElementState::Pressed, _, _) => {
+                    println!("Setting cursor to \"{:?}\"", cursors[cursor_idx]);
+                    window.set_cursor(cursors[cursor_idx]);
+                    if cursor_idx < cursors.len() - 1 {
+                        cursor_idx += 1;
+                    } else {
+                        cursor_idx = 0;
+                    }
+                },
+                _ => (),
+            }
+            
+        }
+        
+    }
+}