Split image-dired.el into several files (part 1/2)

Split image-dired.el into several files (part 1/2)

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.  [this commit]

2) Next, trim down the extra copies to contain only the relevant
   parts.

* lisp/image-dired.el: Move from here...
* 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: ...to here.

* test/lisp/image-dired-tests.el: Move from here...
* test/lisp/image/image-dired-tests.el: ...to here.