Allow different windows to show different pages.

Allow different windows to show different pages.
(doc-view-current-page, doc-view-current-slice, doc-view-current-info)
(doc-view-current-image, doc-view-current-overlay): Remove variables,
add them back as macros instead, using image-mode-winprops instead.
Update all users of those variables.
(doc-view-new-window-function): New function to create a new overlay
for each new window.
(doc-view-mode): Use it and image-mode-setup-winprops.
(doc-view-clone-buffer-hook): Rewrite accordingly.