FLTK only allows 256 different box types, but it doesn't actually check
this when registering new ones.

Move our custom types to a valid range, and add an assert for good
measure to make sure we don't overflow FLTK's internal structures.

Inspired by modern Windows appearance, and to some extent macOS. They
have flat boxes and use white, or very light, colors for interactive
elements. Unfortunately we can't directly control the colors of
widgets, so instead we just lighten everything that uses this box type.

GNOME uses a different design, both their older and newer style. But UI
look is less consistent on Linux, so hopefully our new look is decent
enough there as well.

These are general things and not specific to TigerVNC, so let's move it
to the fltk specific directory for clarity.