Warn when referencing missing images from guides.

Achieving this required extensive refactoring of how the images get
processed.  Previously the existance of images was looked up after
all the @img tag had been replaced.  Now the check for existance is
part of expanding the @img tag, so the check happens also for images
in guides, for which the check didn't occour previously as there is
no images copy step in there.

The old Images class has been replaced with ImageDir and ImageDirSet.