patch 8.1.1966: some code in options.c fits better elsewhere

Problem:    Some code in options.c fits better elsewhere.
Solution:   Move functions from options.c to other files. (Yegappan
            Lakshmanan, closes #4889)
diff --git a/src/proto/window.pro b/src/proto/window.pro
index e685b1c..1c4ade8 100644
--- a/src/proto/window.pro
+++ b/src/proto/window.pro
@@ -82,6 +82,7 @@
 int win_hasvertsplit(void);
 int get_win_number(win_T *wp, win_T *first_win);
 int get_tab_number(tabpage_T *tp);
+char *check_colorcolumn(win_T *wp);
 int win_getid(typval_T *argvars);
 int win_gotoid(typval_T *argvars);
 void win_id2tabwin(typval_T *argvars, list_T *list);