Implement asynchronous thumbnail generation in image-dired

Implement asynchronous thumbnail generation in image-dired

Additionally, all FOO-options defcustoms that were in fact shell command
strings have been converted to argument lists.  Another method for
shrinking PNG thumbs with optipng(1) has been added.
* lisp/image-dired.el: Remove TODO item in commentary.
(image-dired-cmd-create-thumbnail-options):
(image-dired-cmd-create-temp-image-options):
(image-dired-cmd-rotate-thumbnail-options):
(image-dired-cmd-rotate-original-options):
(image-dired-cmd-write-exif-data-options):
(image-dired-cmd-read-exif-data-options): Convert to argument lists.
(image-dired-cmd-pngnq-program, image-dired-cmd-pngcrush-program):
Change string type to file.
(image-dired-cmd-create-standard-thumbnail-command): Remove.
(image-dired-cmd-pngnq-options):
(image-dired-cmd-create-standard-thumbnail-options):
(image-dired-cmd-optipng-program, image-dired-cmd-optipng-options):
New defcustoms.
(image-dired-queue, image-dired-queue-active-jobs):
(image-dired-queue-active-limit): New variables.
(image-dired-pngnq-thumb, image-dired-pngcrush-thumb):
(image-dired-optipng-thumb): New functions.
(image-dired-create-thumb-1): Renamed from image-dired-create-thumb.
Use start-process instead of call-process.  Set file modes.  Trigger
PNG file optimization in process sentinel.
(image-dired-thumb-queue-run, image-dired-create-thumb): New functions.
(image-dired-display-thumbs):
(image-dired-create-thumbs): Don't expect call-process return value.
(image-dired-display-image, image-dired-rotate-thumbnail): Use
start-process instead of call-process.
(image-dired-rotate-original, image-dired-set-exif-data):
(image-dired-get-exif-data): Adapt to arguments being an arg list.