patch 8.1.1406: popup_hide() and popup_show() not implemented yet

Problem:    popup_hide() and popup_show() not implemented yet.
Solution:   Implement the functions.
diff --git a/src/vim.h b/src/vim.h
index 3389ef5..a52ba5e 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -612,7 +612,10 @@
 #define VALID_BOTLINE	0x20	// w_botine and w_empty_rows are valid
 #define VALID_BOTLINE_AP 0x40	// w_botine is approximated
 #define VALID_TOPLINE	0x80	// w_topline is valid (for cursor position)
-#define VALID_POPUP	0x100	// popup has been redrawn
+
+// Values for w_popup_flags.
+#define PFL_HIDDEN	1	// popup is not displayed
+#define PFL_REDRAWN	2	// popup was just redrawn
 
 /*
  * Terminal highlighting attribute bits.