patch 8.1.1553: not easy to change the text in a popup window

Problem:    Not easy to change the text in a popup window.
Solution:   Add popup_settext(). (Ben Jackson, closes #4549)
            Also display a space for an empty popup.
diff --git a/src/version.c b/src/version.c
index f960d52..aa11b7e 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1553,
+/**/
     1552,
 /**/
     1551,