Introduce a WindowProxy for accessing a subset of functionality

from other threads. This currently provides a way for other threads
to wakeup a blocked event loop on X11. Other platforms have stub
functions that need to be implemented. This is similar to
the functionality of glfwPostEmptyEvent.

src/events.rs

@@ -31,6 +31,9 @@ pub enum Event {
 
     /// An event from the mouse has been received.
     MouseInput(ElementState, MouseButton),
+
+    /// The window should be redrawn.
+    Refresh,
 }
 
 pub type ScanCode = u8;