patch 8.1.1441: popup window filter not yet implemented

Problem:    Popup window filter not yet implemented.
Solution:   Implement the popup filter.
diff --git a/src/vim.h b/src/vim.h
index 378fe5f..72bb109 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -615,7 +615,7 @@
 
 // Values for w_popup_flags.
 #define POPF_HIDDEN	1	// popup is not displayed
-#define POPF_REDRAWN	2	// popup was just redrawn
+#define POPF_HANDLED	2	// popup was just redrawn or filtered
 
 /*
  * Terminal highlighting attribute bits.