image-dired: Fix C-a/C-e in thumbnail buffer

image-dired: Fix C-a/C-e in thumbnail buffer

* lisp/image/image-dired.el
(image-dired--movement-ensure-point-pos): New function, and...
(image-dired--movement-command): ...new macros broken out from...
(image-dired-next-line, image-dired-previous-line)
(image-dired-beginning-of-buffer, image-dired-end-of-buffer):
...here.
(image-dired-move-beginning-of-line)
(image-dired-move-end-of-line): New commands.
(image-dired-thumbnail-mode-map): Remap 'move-beginning-of-line'
and 'move-beginning-of-line' to above new commands.