Support for an alternative mechanism for displaying wide characters

Support for an alternative mechanism for displaying wide characters
under X: instead of having two separate fixed-width fonts one of
which is twice the width of the other, you can instead have a single
font in which some characters are twice as wide as others.

This is implemented very simply: if you specify a wide font, it will
be used for wide characters, and if you don't then the normal font
will be used for wide characters (so they'd better _be_ wide in that
font, or there'll be trouble).

I got this idea from Jed, whose latest version supports UTF-8 and
requires a font of this type. If there are going to be X fonts like
that kicking around, there will doubtless be people who want to use
them.

git-svn-id: svn://svn.tartarus.org/sgt/putty@6844 cda61777-01e9-0310-a592-d414129be87e