Allow Info-fontify-maximum-menu-size to specify unlimited

* lisp/info.el (Info-fontify-maximum-menu-size): Fix :type.
Allow t to mean no limit.
(Info-fontify-node): Handle Info-fontify-maximum-menu-size = t.
* lisp/select.el (gui-selection-exists-p-alist): New method.
* lisp/menu-bar.el (menu-bar-edit-menu, clipboard-yank):
* lisp/simple.el (deactivate-mark): Use it.
* lisp/term/x-win.el (gui-selection-exists-p):
* lisp/term/w32-win.el (gui-selection-exists-p):
* lisp/term/pc-win.el (gui-selection-exists-p):
* lisp/term/ns-win.el (gui-selection-exists-p): Provide a backend instance.