Slightly robustify sly-sticker's face calculation

* sly-stickers.el (sly-stickers-max-nested-stickers): New
user-visible customization variable.
(sly-stickers-color-face-attribute): New user-visible variable.
(sly-stickers--guess-face-color): New helper.
(sly-stickers--set-face): Use new helpers and variables.