patch 8.1.0307: there is no good way to get the window layout

Problem:    There is no good way to get the window layout.
Solution:   Add the winlayout() function. (Yegappan Lakshmanan)
diff --git a/src/evalfunc.c b/src/evalfunc.c
index 2bebdd1..5e7c013 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -463,6 +463,7 @@
 static void f_winbufnr(typval_T *argvars, typval_T *rettv);
 static void f_wincol(typval_T *argvars, typval_T *rettv);
 static void f_winheight(typval_T *argvars, typval_T *rettv);
+static void f_winlayout(typval_T *argvars, typval_T *rettv);
 static void f_winline(typval_T *argvars, typval_T *rettv);
 static void f_winnr(typval_T *argvars, typval_T *rettv);
 static void f_winrestcmd(typval_T *argvars, typval_T *rettv);
@@ -952,6 +953,7 @@
     {"winbufnr",	1, 1, f_winbufnr},
     {"wincol",		0, 0, f_wincol},
     {"winheight",	1, 1, f_winheight},
+    {"winlayout",	0, 1, f_winlayout},
     {"winline",		0, 0, f_winline},
     {"winnr",		0, 1, f_winnr},
     {"winrestcmd",	0, 0, f_winrestcmd},
@@ -13743,6 +13745,29 @@
 }
 
 /*
+ * "winlayout()" function
+ */
+    static void
+f_winlayout(typval_T *argvars, typval_T *rettv)
+{
+    tabpage_T	*tp;
+
+    if (rettv_list_alloc(rettv) != OK)
+	return;
+
+    if (argvars[0].v_type == VAR_UNKNOWN)
+	tp = curtab;
+    else
+    {
+	tp = find_tabpage((int)get_tv_number(&argvars[0]));
+	if (tp == NULL)
+	    return;
+    }
+
+    get_framelayout(tp->tp_topframe, rettv->vval.v_list, TRUE);
+}
+
+/*
  * "winline()" function
  */
     static void