Add an API to get the binding of values

The bindings of families in a match or sort pattern are important in the
selection of default fonts for each language.

Currently the only way to determine these bindings is to use FcFont(Set)Sort
or FcFont(Set)Match.  Providing an API to get the bindings would help
applications writing their own sort/match functions that may have different
priorities for the properties or have additional information that can be used
to optimize the sort/match.

I guess FcValueGetBinding is not really feasible as FcValue is a public
struct and FcValueGetBinding would seem a bit strange against
FcPatternAdd(,,FcValue,) and FcPatternAddWeak(,,FcValue,).

FcPatternGetBinding(FcPattern *p, const char *object, int id,
                    FcValueBinding *b)
may be an option.

Another option would be
FcPatternGetWithBinding(FcPattern *p, const char *object, int id,
                        FcValue *v, FcValueBinding *b)
which would save the duplicate work of finding the (private) FcValueList
twice.