Improve selection of fonts available from `mouse-set-font'

Improve selection of fonts available from `mouse-set-font'

People get confused on a build without font dialogs (such as a
Lucid build) if `menu-set-font' and `mouse-set-font' don't
present them a list of the fonts actually available on their
system.

* lisp/mouse.el (mouse-generate-font-name-for-menu)
(mouse-generate-font-menu): New functions.
(mouse-select-font): Allow the user to select from all fonts
available on the system.
(mouse-set-font): Use `mouse-select-font' to display font menu.