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.
Fix event name
Add mac implementation of wakeup_event_loop.
Change todo to unimplemented
Merge pull request #161 from glennw/thread-proxy

Introduce a WindowProxy for accessing a subset of functionality