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/version.c b/src/version.c
index a59803b..ad09b56 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1559,
+/**/
     1558,
 /**/
     1557,