playlist/registry: add option "as_directory"

This allows users to disable the "CUE files as directories" feature
without having to disable the CUE playlist plugin completely.  This
feature has been annoying some users.

src/db/update/Playlist.cxx

@@ -95,7 +95,7 @@ UpdateWalk::UpdatePlaylistFile(Directory &directory,
 	if (plugin == nullptr)
 		return false;
 
-	if (plugin->as_folder)
+	if (GetPlaylistPluginAsFolder(*plugin))
 		UpdatePlaylistFile(directory, name, info, *plugin);
 
 	PlaylistInfo pi(name, info.mtime);

src/db/update/SpecialDirectory.cxx

@@ -52,7 +52,16 @@ static bool
 HavePlaylistPluginForFilename(const char *filename) noexcept
 {
 	const char *suffix = PathTraitsUTF8::GetFilenameSuffix(filename);
-	return suffix != nullptr && playlist_suffix_supported(suffix);
+	if (suffix == nullptr)
+		return false;
+
+	const auto plugin = FindPlaylistPluginBySuffix(suffix);
+	if (plugin == nullptr)
+		return false;
+
+	/* discard the special directory if the user disables the
+	   plugin's "as_directory" setting */
+	return GetPlaylistPluginAsFolder(*plugin);
 }
 
 bool