Dialog and Menu Background color transparency

[dgud]

A year ago it wasn't possible to do transparency in wx, so I removed the possibilty to change that in the dialog.

Since then I have added that functionality wx, seen when doing drag window before attaching the windows.
I have not had time to investigate if we can make the menus transparent on windows, on the Mac it just works.

I can take a look when I have time but it is very low priority on my list.
And the windows native dialog can do alpha values, you will have use wings builtin color dialog for that.