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/popupwin.pro b/src/proto/popupwin.pro
index a251f9a..7733368 100644
--- a/src/proto/popupwin.pro
+++ b/src/proto/popupwin.pro
@@ -11,7 +11,9 @@
 void f_popup_clear(typval_T *argvars, typval_T *rettv);
 void f_popup_create(typval_T *argvars, typval_T *rettv);
 void f_popup_atcursor(typval_T *argvars, typval_T *rettv);
+void f_popup_beval(typval_T *argvars, typval_T *rettv);
 void popup_close_for_mouse_click(win_T *wp);
+void popup_handle_mouse_moved(void);
 void f_popup_filter_menu(typval_T *argvars, typval_T *rettv);
 void f_popup_filter_yesno(typval_T *argvars, typval_T *rettv);
 void f_popup_dialog(typval_T *argvars, typval_T *rettv);