Split out general checks into its own verifier class (and remove static usage)

osu.Game/Rulesets/Edit/BeatmapVerifier.cs

@@ -0,0 +1,24 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using System.Collections.Generic;
+using System.Linq;
+using osu.Game.Beatmaps;
+using osu.Game.Rulesets.Edit.Checks;
+using osu.Game.Rulesets.Edit.Checks.Components;
+
+namespace osu.Game.Rulesets.Edit
+{
+    /// <summary>
+    /// A ruleset-agnostic beatmap converter that identifies issues in common metadata or mapping standards.
+    /// </summary>
+    public class BeatmapVerifier : IBeatmapVerifier
+    {
+        private readonly List<ICheck> checks = new List<ICheck>
+        {
+            new CheckBackground(),
+        };
+
+        public IEnumerable<Issue> Run(IBeatmap beatmap) => checks.SelectMany(check => check.Run(beatmap));
+    }
+}