(artist-version): 1.2.6

(artist-version): 1.2.6
(artist-prev-next-op-alist): New variable.
(artist-select-next-op-in-list): New function.
(artist-select-prev-op-in-list): New function.
(artist-make-prev-next-op-alist): New function.
(artist-is-in-op-list-p): New function.
(artist-get-first-non-nil-op): New function.
(artist-get-last-non-nil-op): New function.
(artist-mode-init): Call artist-get-last-non-nil-op to initialize
   artist-prev-next-op-alist.
(artist-mode-map): Add binding for C-mouse-4 and C-mouse-5 to
   change drawing operation.