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/evalfunc.c b/src/evalfunc.c
index d371c28..8c8bb4d 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -822,6 +822,7 @@
     {"popup_hide",	1, 1, f_popup_hide},
     {"popup_move",	2, 2, f_popup_move},
     {"popup_notification", 2, 2, f_popup_notification},
+    {"popup_settext",	2, 2, f_popup_settext},
     {"popup_show",	1, 1, f_popup_show},
 #endif
 #ifdef FEAT_FLOAT