Made a bunch of changes to the overall layout of the code, native Win32 code should now be less distracting.
Added Line.h and Window.h.

* There is a now a class for Lines or ("walls"), to simplify making levels on the fly.

* There is a problem with energy being lost during impulses with walls due to the next frames acceleration not being accounted for.

* Angular forces are missing.

Signed-off-by: Peter Mårtensson <veritas.is.me@gmail.com>