GTK font selector

[Chong Yidong]

I've installed a couple of font-related changes to CVS.  The font parser
will now attempt to understand GTK-style font names.  It tries to guess
whether a font name is in the GTK format ("Monospace Bold 9") or the
Fontconfig format ("Monospace-9:bold").

I *think* it shouldn't be possible to confuse the parser for any
real-world font names, but it would be good if people could test this.

Also, now that Emacs can understand GTK font names, I've changed the
Options->Save Options menu entry so that, when running under GTK with
fontconfig support, it pops up a GTK font selector instead of Emacs'
usual font menu.  The selected font is applied to the `default' face on
all frames, via Customize.

Please let me know if there are any problems.