Rename UI_UPDATE as MOVE_UI_UPDATE

All the other constants named UI_* are special key names that can be
passed to midend_process_key(), but UI_UPDATE is a special return value
from the back-end interpret_move() function instead.  This renaming
makes the distinction clear and provides a naming convention for future
special return values from interpret_move().

singles.c

@@ -1533,7 +1533,7 @@ static char *interpret_move(const game_state *state, game_ui *ui,
             action = TOGGLE_CIRCLE;
         }
     }
-    if (action == UI) return UI_UPDATE;
+    if (action == UI) return MOVE_UI_UPDATE;
 
     if (action == TOGGLE_BLACK || action == TOGGLE_CIRCLE) {
         i = y * state->w + x;