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/proto/window.pro b/src/proto/window.pro
index 7ed8042..c31e599 100644
--- a/src/proto/window.pro
+++ b/src/proto/window.pro
@@ -94,4 +94,5 @@
 win_T *win_id2wp(typval_T *argvars);
 int win_id2win(typval_T *argvars);
 void win_findbuf(typval_T *argvars, list_T *list);
+void get_framelayout(frame_T *fr, list_T *l, int topframe);
 /* vim: set ft=c : */