Add an option to change the highlighting background color of `updatemenus`

Currently only the normal (i. e. unhighlighted) background color can be changed through the parameter bgcolor. If you set this background to a dark coloring, you most probaby also want to adjust the text font to a bright coloring so that the text is still readable. But then the text becomes pretty unreadable while being highlighted since the highlighting background color is very bright!

Therefore I think an additional parameter bgcolor_active or the like should be introduced.

Additionally, an even more sophisticated solution could involve an automatic adjustment of the text font color and/or background color depending on the color brightness of the user defined bgcolor and/or font.color values (this seems to be already the case with the font color of hoverinfo labels).

Comienza con las entradas de referencia de layout updatemenus para bgcolor y font.color y, después, sigue el manejo existente del estado resaltado. Decide si la opción de fondo activo solicitada es el cambio acotado; se considera terminado cuando la opción está documentada y los controles resaltados siguen siendo legibles con fondos normales oscuros, con cobertura para el nuevo comportamiento.