merge window and window_24bit together

[MCWertGaming]

We need to merge the Term::Window and Term::WIndow_24bit classes some day to avoid having the same code twice (and fix things twice too). The first idea of:

struct Color {
    enum { bit24, bit3, bit4 } tag;
    union {
        struct {
            char R, G, B;
        };   
        fg color_fg;
        fgB color_fgB;
    };  
};   

Didn't worked, because of the vectors and forbidden anonymous structures and unions. A different approach is required here.