[rar] add a callback class for unrarlib rather than calling into guilib

We don't want to call into guilib directly from unrarlib, so this does the same as what we were doing (though with a 200ms delay) via a callback class.

@mkortstiege This should be pretty equivalent to what we already have in master.  i.e. it is useful as a base from which to make further changes, such as increasing the time before display, or switching to busy dialog etc.
heh, we probably want cancel(false); here :)