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.

[GrahamNZ]

By the way, the tabtest gui example file isn't currently working in Windows (fpGUI 2.0.3). The tabs aren't resizing for the text:

[graemeg]

Umm, I see the same problem in Linux, using the tabtest. Looking at your screenshot, it also looks like it's using the wrong font for the tabs, versus the font used in the tabsheet itself.