Teach image-dired to also generate large thumbs

Teach image-dired to also generate large thumbs

* lisp/image-dired.el (image-dired-thumbnail-storage): Add
standard-large option.
(image-dired-thumb-size): Add condition for standard-large storage.
(image-dired-insert-thumbnail): Check for new option.  Change
thumbnail path conditionally.
(image-dired-thumb-size): New function.
(image-dired-create-thumb, image-dired-line-up-dynamic): Use it.