Provide a native font selection dialog or a pure Tcl substitute.

Currently this isn't quite as automatic as I'd like.

Build on Windows:
 tclkitsh critcl.kit -lib choosefont.tcl choose_w32.tcl
 copy choosefont.dll ix86-win32\choosefont.dll

Build on Linux (Gtk+):
 critcl -lib choosefont.tcl choose_gtk.tcl
 cp choosefont.so ix86-linux/choosefont.so

