Change (add?) <F> command for handling function buttons

[FrightRisk]

First, do we need DCC++ Classic compatibility for Fn buttons? If not, we can really simplify the code by using the DCC++ EX command instead of the command (upper case vs. lower). If we do need it, perhaps we can make that a unit and then call or based on parsing the command and getting the version or looking for "EX"

[matt-vdv]

@FrightRisk I think we should be ok to drop backwards compatibility, as long as we show a message to DCC++ Classic users telling them why it's not working and where they can get the new software from.

Do you know of any reasons someone may want to stay on DCC Classic (features that aren't in Command station)? If there are, we may need to rethink...