Add support for custom backcolor to grid's headers

[fyurisich]

Add support for custom back-color to grid's headers.

[fyurisich]

To implement these modification we must add the necessary logic to paint the background at function TGRID_HEADER_CUSTOMDRAW in h_grid.prg

[fyurisich]

See https://stackoverflow.com/questions/3904572/c-sharp-listview-colum-header-height-windows-form