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/x11/mod.rs

@@ -2,7 +2,7 @@
 pub use self::headless::HeadlessContext;
 
 #[cfg(feature = "window")]
-pub use self::window::{Window, MonitorID, get_available_monitors, get_primary_monitor};
+pub use self::window::{Window, WindowProxy, MonitorID, get_available_monitors, get_primary_monitor};
 
 mod ffi;