Support for custom value font

[thijstriemstra]

I'd like to use a (custom) bold font for values. the big font appears to correspond to menu items, the small font to menu header?

// Set custom fonts and specify width, height and baselineOffset of characters
menu.setFontBig(&Piboto_Regular12pt7b, 8, 12, 12).setFontSmall(&Piboto_Bold12pt7b, 8, 12, 12);

[Spirik]

If I find mono-spaced font with 6x12px characters that different enough from default one (for compatibility with default scaling and sizes of the GEM), I may consider adding this feature. Giving that it won't blow up library size too much using current architecture, of course. I still very much have to consider space and performance limitations of AVR-boards, so I try to avoid feature bloat as much as possible (and partially because of that Advanced Mode exists, so that users can override and extend GEM functionality as they seem fit for their specific requirements without the need to edit source files directly).