retrieve content area only once
change exit hotkey

Alt + Ctrl + Q interferes with @
fix iteration bug

Actually the same bug as in commit d5c54f689fa,
but that commit only fixed stepping forward.
Fix stepping backwards as well now.
remove old obsolete method
create all parts at once

All parts are created by the CreateAllParts() function in misc.ahk,
called by Main.ahk after disabling window redrawing. Do not attempt
to create them already in the PresentationWindow constructor.