window now correctly remembers size, position, fullscreen, maximised state from previous session.
this was currently prevented by the crash on exit. https://github.com/techartorg/bqt/issues/47
removed atexit module and on_exit method. since we could simplify the code
did not solve the crash on exit, for further debate on this see above linked issue
atexit
module andon_exit
method. since we could simplify the codenote