* lisp/mwheel.el: Don't use `custom-initialize-delay`.

* lisp/mwheel.el: Don't use `custom-initialize-delay`.

Set up the keybindings when loading the file.

(mouse-wheel--installed-bindings-alist): Move to beginning.
(mouse-wheel-change-button): Don't update bindings when they're not installed.
(mouse-wheel--setup-bindings): New function extracted from `mouse-wheel-mode`.
(<topleve>): Call it when loading the file.
(mouse-wheel-mode): Use the default :initializer.