* lisp/doc-view.el: Add support for DjVu.

* lisp/doc-view.el: Add support for DjVu.
(doc-view-djvu->png-converter-function): New config var.
(doc-view-single-page-converter-function, doc-view--image-type)
(doc-view--image-file-extension): New vars.
(doc-view-mode): Initialize them.
(doc-view-goto-page): Use them.
(doc-view-mode-p): Add support for ddjvu.
(doc-view-djvu->png-converter-ddjvu, doc-view-djvu->png-1)
(doc-view-set-up-single-converter): New funs.
(doc-view-pdf/ps->png): Extend for djvu.
(doc-view-document->png): Rename from doc-view-pdf->png.
(doc-view-convert-current-doc): Handle djvu.
(doc-view-insert-image, doc-view-display)
(doc-view-already-converted-p): Don't hardcode png.
(doc-view-set-doc-type): Recognize djvu docs.

Fixes: debbugs:13164