Mouse-wheel scrolling for DocView Continuous mode. (Bug#4896)

Mouse-wheel scrolling for DocView Continuous mode.  (Bug#4896)

* mwheel.el (mwheel-scroll-up-function)
(mwheel-scroll-down-function): New defvars.
(mwheel-scroll): Funcall `mwheel-scroll-up-function' instead of
`scroll-up', and `mwheel-scroll-down-function' instead of
`scroll-down'.

* doc-view.el (doc-view-scroll-up-or-next-page)
(doc-view-scroll-down-or-previous-page): Add optional ARG.
Use this ARG in the call to image-scroll-up/image-scroll-down.
Change `interactive' spec to "P".  Goto next/previous page only
when `doc-view-continuous-mode' is non-nil or ARG is nil (for the
SPC/DEL case).  Doc fix.
(doc-view-next-line-or-next-page)
(doc-view-previous-line-or-previous-page): Rename arg to ARG
for consistency.
(doc-view-mode): Set buffer-local `mwheel-scroll-up-function' to
`doc-view-scroll-up-or-next-page', and buffer-local
`mwheel-scroll-down-function' to
`doc-view-scroll-down-or-previous-page'.