(thumbs-extra-images): New variable. Make it buffer-local

(thumbs-extra-images): New variable.  Make it buffer-local
and permanent-local.
(thumbs-max-image-number): New variable.  Make it
(thumbs-do-thumbs-insertion): Use them
(thumbs-per-line): Change default to 4.
(thumbs-marked-list): Rename from thumbs-markedL.
(thumbs-cleanup-thumbsdir, thumbs-delete-images)
(thumbs-rename-images): Use -list instead of L for internal variables.
(thumbs-call-convert): Use call-process instead of shell-command.
(thumbs-insert-thumb): Add filename as help-echo to each image.
(thumbs-show-from-dir): Rename from thumbs-show-all-from-dir.
Give dir to thumbs-show-thumbs-list.
(thumbs-show-thumbs-list): Set default-directory to that of images.
(thumbs-dired-show): Rename from thumbs-dired-show-all.
(thumbs-display-thumbs-buffer, thumbs-show-more-images): New functions.
(thumbs-mode-map): Bind "+" to thumbs-show-more-images.
(thumbs-view-image-mode-map): Bind "^" to thumbs-display-thumbs-buffer.