Add background (`bg`) attribute to table of boxtypes

This is a first step to fix some background (re-)drawing issues in
widgets. A new bit in the array of boxtypes can be used to determine
if a particular boxtype uses a solid background (e.g. all FL_*_BOX
types) or if the parent widget is responsible for drawing it (FL_NO_BOX
and all FL_*_FRAME) types, and maybe more ...

The old struct member `set` in the struct of boxtypes has been
renamed to `flags` and is now used as a bit field.

Except these changes, this first commit fixes the focus box drawing
of specific boxtypes, as seen in unittest_schemes.cxx in the
Fl_Check_Button with label "Check", and very likely more.

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