patch 8.1.1559: popup window title property not implemented yet

Problem:    Popup window title property not implemented yet.
Solution:   Implement the title property.
diff --git a/src/window.c b/src/window.c
index ee616ee..7f8bb78 100644
--- a/src/window.c
+++ b/src/window.c
@@ -4857,6 +4857,7 @@
     free_callback(&wp->w_filter_cb);
     for (i = 0; i < 4; ++i)
 	VIM_CLEAR(wp->w_border_highlight[i]);
+    vim_free(wp->w_popup_title);
 #endif
 
 #ifdef FEAT_SYN_HL