Add suitable ssc font shape change rules

[FrankMittelbach]

Brief outline of the enhancement

The kernel conceptually supports ssc (spaced small caps) as a shape, something that is available in some commercial fonts and can also be artificially produced from existing otf files using otftotfm.

What is missing until now are any \DeclareFontShapeChangeRule declarations to really make use of that special shape.

Minimal example showing the current behaviour

\sscshape\itshape  % currently produces  "it"
\itshape\sscshape  % currently produces  "ssc" (if available else "n")

etc

Minimal example showing the desired new behaviour

\sscshape\itshape  % produces "sscit"
\itshape\sscshape  % produces "sscit"

[FrankMittelbach]

Also took the opportunity (since there is anyway a rollback) to provide better fallbacks for the sw shape, i.e., falling back to it is an sw combination is not available.