app: rename gimpimageviewable.[ch] to gimpimageproxy.[ch]

... in preparation for actually renaming the class, but as a
separate step to keep the history.

(cherry picked from commit 0ddd3330b8)

app/core/Makefile.am

@@ -321,10 +321,10 @@ libappcore_a_sources = \
 	gimpimage-undo.h			\
 	gimpimage-undo-push.c			\
 	gimpimage-undo-push.h			\
+	gimpimageproxy.c			\
+	gimpimageproxy.h			\
 	gimpimageundo.c				\
 	gimpimageundo.h				\
-	gimpimageviewable.c			\
-	gimpimageviewable.h			\
 	gimpimagefile.c				\
 	gimpimagefile.h				\
 	gimpitem.c				\