Fix misspelled symbols in artist.el

Fix misspelled symbols in artist.el

* lisp/textmodes/artist.el (artist-key-undraw-continuously)
(artist-key-draw-continuously)
(artist-key-do-continuously-continuously)
(artist-key-do-continuously-poly)
(artist-key-do-continuously-1point)
(artist-key-do-continuously-2points)
(artist-key-do-continuously-common)
(artist-key-set-point-continuously)
(artist-mouse-draw-continuously): Rename functions to fix misspelling of
"continuously".  Leave old function names as obsolete aliases and update
all callers.
(artist-mt, artist-go-get-undraw-fn, artist-go-get-interval-fn)
(artist-go-get-fill-pred, artist-go-get-fill-fn)
(artist-key-undraw-common, artist-key-draw-common)
(artist-key-do-continously-common, artist-key-set-point-common)
(artist-down-mouse-1): Rename symbol to 'artist-do-continuously' to fix
misspelling of "continuously".  Continue supporting the old symbol for
backwards-compatibility.