Split image-dired.el into several files (part 2/2)

Split image-dired.el into several files (part 2/2)

Use a git trick to split a file while preserving line history (for
"git blame", "git log --follow", etc.):

1) Make exact copies of the original file, in the same commit as
   moving it.

2) Next, trim down the extra copies to contain only the relevant
   parts.  [this commit]

* lisp/image-dired.el:
* lisp/image/image-dired-dired.el:
* lisp/image/image-dired-external.el:
* lisp/image/image-dired-tags.el:
* lisp/image/image-dired-util.el:
* lisp/image/image-dired.el: Trim files down to keep only one copy of
each definition.