patch 8.1.1645: cannot use a popup window for a balloon

Problem:    Cannot use a popup window for a balloon.
Solution:   Add popup_beval().  Add the "mousemoved" property.  Add the
            screenpos() function.
diff --git a/src/proto/move.pro b/src/proto/move.pro
index ed45c4d..c2ec8d5 100644
--- a/src/proto/move.pro
+++ b/src/proto/move.pro
@@ -27,6 +27,7 @@
 int win_col_off2(win_T *wp);
 int curwin_col_off2(void);
 void curs_columns(int may_scroll);
+void f_screenpos(typval_T *argvars, typval_T *rettv);
 void scrolldown(long line_count, int byfold);
 void scrollup(long line_count, int byfold);
 void check_topfill(win_T *wp, int down);