Switch to game_mkhighlight for bevel & contrast colors (guess, mines, samegame)

Use the existing, handy utility function for consistent coloring.
Change game_mkhighlight to preserve hue; add rgb/hsl conversion C utils.

Been experimenting with some non-gray background colors, and the RGB-space lighten/darken math in game_mkhighlight was often not entirely pleasing, due to the hue shifts it could introduce. Altering lightness in HSL tends to work better.

(Might also want to try desaturating the highlight/lowlight colors.)
Hide debug panel by default; show with ?debug querystring
Index page: Clarify supported browsers in tip
In which we discover options beyond default Bootstrap and #ccc puzzle backgrounds.