Fix issue where key window would lose all mouse events once mouse left
that window.
Make sure that only window under mouse receives mouse scroll wheel
events.
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.
