Add sort columns to filter parser

Also pass the filter column enum through to filter tree instead of string.

src/filterparser/filtercolumn.h

@@ -0,0 +1,53 @@
+/*
+ * Strawberry Music Player
+ * Copyright 2026, Jonas Kvinge <jonas@jkvinge.net>
+ *
+ * Strawberry is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Strawberry is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Strawberry.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#ifndef FILTERCOLUMN_H
+#define FILTERCOLUMN_H
+
+enum class FilterColumn {
+  Unknown,
+  Title,
+  TitleSort,
+  Album,
+  AlbumSort,
+  Artist,
+  ArtistSort,
+  AlbumArtist,
+  AlbumArtistSort,
+  Composer,
+  ComposerSort,
+  Performer,
+  PerformerSort,
+  Grouping,
+  Genre,
+  Comment,
+  Filename,
+  URL,
+  Track,
+  Year,
+  Samplerate,
+  Bitdepth,
+  Bitrate,
+  Playcount,
+  Skipcount,
+  Length,
+  Rating,
+};
+
+#endif // FILTERCOLUMN_H