Allow changing CopyFormat with BeforeAutoFill event

[jonas-engberg-matsmart-se]

Fixes #212

[anmcgrath]

Thanks @jonas-engberg-matsmart-se

Sorry to be a pain but I think that BeforeAutoFill should be an event so that it can't be invoked outside of the Sheet class.

(reviewing this code I realised that I did the same thing with ScreenUpdatingChanged and will need to change that).

Potentially this could be done by having an internal function in Sheet e.g EmiteBeforeAutoFill().

[jonas-engberg-matsmart-se]

Yes, agreed. Added an internal EmitBeforeAutoFill to the Sheet class.

[anmcgrath]

Thank @jonas-engberg-matsmart-se appreciate it!