Remove all non-public API from XKBcommon.h header

Remove all non-public API from XKBcommon.h header

The noble intention was to expose all the new API and new generic types
in the split out kbproto headers through XKBcommon.h. It turns out that
would be a massive amount of work in the server. Someday, but first just
wedging in XkbCompileKeymap* would be good.

Most of the API is in new internal xkb*.h headers. In order to allow the
XKBcommon.h header to be used from the server, we can't pull in other
headers from kbproto since the server has its own copies. However, types
that are different (XkbDescRec, XkbAction) still have Xkbc equivalents
here, and I think they should be used in the server.