Rewrite listing code to generate XkbComponentListPtr

Rewrite listing code to generate XkbComponentListPtr

In xkbcomp, the listing code printed out the xkb files to stdout and the
caller would parse the output. Here, we can just generate a
XkbComponentListPtr and pass it back.

This should be a series of smaller commits, but there was no clean way to
do it since it's basically a complete rewrite except for the core map
matching logic.

A lot of code used for special printing modes in xkbcomp has been
shedded. Callers can massage the output as they please.