Copy images as part of the install-info make target

By default, install-info uses a symlink to html documentation for included images. This results in broken links unless the user also runs make install-WWW, which is unlikely since there is already a target for exactly that combination: install-doc.

Update install-info to copy images, but keep the old behaviour when install-doc is run (because the symlink will work in that case and there is no need to copy duplicate images).

Closes #6110 (closed)