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.

Follow the colors that modern desktops use when it comes to background,
text and selections.

There is something broken with these FLTK draw routines on Windows. They
leave gaps at the start and end of the arc/pie rather than filling the
whole specified span. So we need to nudge the numbers a bit to work
around this.

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.

This was not meant to be included in the previous commit. The static
storage for the font string is more than enough.

Fl::set_font() doesn't keep its own copy, so we need to make sure the
font string doesn't get freed or overwritten at a later point.

Use the font specified by the system for UI elements. For Windows and
macOS this is straight forward, but Linux is more complex as there is no
single source for this information.

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