New feature: set optimal slice from BoundingBox information.

New feature: set optimal slice from BoundingBox information.
* doc-view.el (doc-view-mode-map): Add keybinding.
(doc-view-menu): Add menu entry.
(doc-view-set-slice): Adapt docstring.
(doc-view-get-bounding-box, doc-view-guess-paper-size)
(doc-view-scale-bounding-box)
(doc-view-set-slice-from-bounding-box): New functions.
(doc-view-paper-sizes): New defvar.

* doc/emacs/misc.texi (DocView Slicing): Document new slice from
BoundingBox feature.