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).

先从 layout updatemenus 中关于 bgcolor 和 font.color 的参考条目开始，然后追踪现有的高亮状态处理。确定所请求的活动背景选项是否属于范围明确的更改；完成的标准是该选项已记录在文档中，并且在普通背景较暗时高亮控件仍保持可读，同时为新行为提供覆盖。