[rar] add a callback class for unrarlib rather than calling into guilib direct from UnrarXLib
[progress] make CGUIDialogProgressBar::ShowProgressBar/SetCanCancel thread-safe without messing about with graphicscontext
Merge pull request #5000 from jmarshallnz/hacky_rar_callback

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