Add TabVisible property so that tabs can be hidden completely rather than just disabled

[GrahamNZ]

It's great to see FPgui come back to life! Thank you Graeme.

One of the changes I made to my copy of 1.4.1 was to add TabVisible as a boolean property to TfpgTabSheet so that tabs can be completely hidden. My code isn't very good as I got confused about how the left and right margins worked so I won't try to upload it as a pull request (not that I know how to, anyway!), but it seems to work for me. Basically all the loops with

        while h <> nil do
        begin
           ....
        end; 

become

        while h <> nil do
        begin
          if h.TabVisible
          then
             ....
          end;
       end;

but, as I said, this changes how the margins are incremented too.

It would be great if this was added to the official release.

I know some people think tabs should only be disabled as part of a UI and not removed when not needed but there are times when I think this option makes a screen much clearer.