Allow face-remapping using :font, and use it in mouse-appearance-menu.

Allow face-remapping using :font, and use it in mouse-appearance-menu.

* mouse.el (mouse-appearance-menu): If x-select-font returns a
font spec, set the font directly.

* xfaces.c (merge_face_vectors): If the target font specfies a
font spec, make the font's attributes take precedence over
directly-specified attributes.
(merge_face_ref): Recognize :font.

Fixes: debbugs:3228