invariants
This also removes the need for "box"ing the callback in favour of
storing a raw `*mut` pointer. We can do this by ensuring that we never
store the pointer for longer than the lifetime of the user callback,
which is the duration of a call to `poll_events` or `run_forever`.
Also removes old commented out event code from the window module.
This is a follow up to the new API introduced in #20.
This also fixes the issue where window resize events would not be
emitted until the end of the resize. This PR fixese #39 by ensuring that
the user callback given to either `EventsLoop::poll_events` or
`EventsLoop::run_forever` can be called by each window delegate's resize
callback directly.
