Face remapping on mouse menu

[Chong Yidong]

Currently, shift-down-mouse-1 pops up a menu that lets the user choose a
different font to apply to the current frame.

This doesn't seem like a very useful operation for a mouse binding.
Also, the choices aren't very suitable when fontconfig is in use; the
menu consists of a pre-generated list of non-fontconfig fonts
(x-fixed-font-alist) plus whichever fontconfig fonts happen to be loaded
(generate-fontset-menu).

Now that the face-remapping code has been checked in, I'd suggest
changing this menu to a "buffer faces" menu.  It should contain a few
items that remap the `default' face in the current buffer: an item to
increase the face size, an item to decrease the face size, and an item
to change the face font.  When fontconfig is available, the latter can
use x-popup-dialog to pop up the GTK font selector, instead of relying
on x-fixed-font-alist.

WDYT?