patch 8.1.1560: popup window hidden option not implemented yet

Problem:    Popup window hidden option not implemented yet.
Solution:   Implement the hidden option.
diff --git a/src/popupwin.c b/src/popupwin.c
index 3603a1c..c042fe8 100644
--- a/src/popupwin.c
+++ b/src/popupwin.c
@@ -428,6 +428,13 @@
 	    semsg(_(e_invarg2), tv_get_string(&di->di_tv));
     }
 
+    nr = dict_get_number(dict, (char_u *)"hidden");
+    if (nr > 0)
+    {
+	wp->w_popup_flags |= POPF_HIDDEN;
+	--wp->w_buffer->b_nwindows;
+    }
+
     popup_mask_refresh = TRUE;
 }