refactor - introduce contest-specific message handling

[w7sst]

Added new virtual method to TContest.SendMsg(). This is derived by those contests with contest-specific messaging. Codes was moved from its old location into the new methods. No functionality was changed - only code movement. This fixes #184.

I like this change. It removes lots of case statements. Here is an example from ARRL Field Day...

{
  Overrides TContest.SendMsg() to send contest-specific messages.

  Adding a contest: TContest.SendMsg(AMsg): send contest-specfic messages
}
procedure TArrlFieldDay.SendMsg(const AStn: TStation; const AMsg: TStationMessage);
begin
  case AMsg of
    msgCQ: SendText(AStn, 'CQ FD <my>');
    msgNrQm:
      case Random(5) of
        0,1: SendText(AStn, 'NR?');
        2: SendText(AStn, 'SECT?');
        3: SendText(AStn, 'CLASS?');
        4: SendText(AStn, 'CL?');
      end;
    msgLongCQ: SendText(AStn, 'CQ CQ FD <my> <my> FD');  // QrmStation only
    else
      inherited SendMsg(AStn, AMsg);
  end;
end;

Note - I added many people to this review to keep people informed of the design change. My next step is to integrate the SST Contest.