patch 8.1.1138: plugins don't get notified when the popup menu changes

Problem:    Plugins don't get notified when the popup menu changes.
Solution:   Add the CompleteChanged event. (Andy Massimino. closes #4176)
diff --git a/src/popupmnu.c b/src/popupmnu.c
index 7be8a9b..ba9c7ad 100644
--- a/src/popupmnu.c
+++ b/src/popupmnu.c
@@ -923,6 +923,22 @@
     return pum_height;
 }
 
+/*
+ * Add size information about the pum to "dict".
+ */
+    void
+pum_set_event_info(dict_T *dict)
+{
+    if (!pum_visible())
+	return;
+    dict_add_number(dict, "height", pum_height);
+    dict_add_number(dict, "width", pum_width);
+    dict_add_number(dict, "row", pum_row);
+    dict_add_number(dict, "col", pum_col);
+    dict_add_number(dict, "size", pum_size);
+    dict_add_special(dict, "scrollbar", pum_scrollbar ? VVAL_TRUE : VVAL_FALSE);
+}
+
 # if defined(FEAT_BEVAL_TERM) || defined(FEAT_TERM_POPUP_MENU) || defined(PROTO)
     static void
 pum_position_at_mouse(int min_width)