Decouple bar line hitobjects from generator

Introduce an IBarLine interface, which together with generic constraints
helps decouple BarLineGenerator from the actual hitobject types it
creates. Thanks to this, all rulesets that want bar lines can provide
an implementation of IBarLine that also derives from the base hitobject
class.

This allows DrawableBarLines in taiko and mania to be migrated back to
DrawableTaikoHitObject and DrawableManiaHitObject base classes
respectively. This in turn resolves #6215 without code duplication,
since the missing anchoring application is now done in mania's
DrawableBarLine through deriving from DrawableManiaHitObject.

osu.Game.Rulesets.Taiko/Objects/BarLine.cs

@@ -0,0 +1,12 @@
+// 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 osu.Game.Rulesets.Objects;
+
+namespace osu.Game.Rulesets.Taiko.Objects
+{
+    public class BarLine : TaikoHitObject, IBarLine
+    {
+        public bool Major { get; set; }
+    }
+}