Remove AvailableColumns from ManiaRulesetContainer

Also restructures with the addition of a ManiaBeatmap which holds definitions for "groups" of columns. At the moment these are empty save for a "Column" property, but can be expanded in the future, maybe.

osu.Game.Rulesets.Mania/Beatmaps/ManiaBeatmap.cs

@@ -0,0 +1,33 @@
+// Copyright (c) 2007-2017 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+using System.Collections.Generic;
+using System.Linq;
+using osu.Game.Beatmaps;
+using osu.Game.Rulesets.Mania.Objects;
+using osu.Game.Rulesets.Mania.UI;
+
+namespace osu.Game.Rulesets.Mania.Beatmaps
+{
+    public class ManiaBeatmap : Beatmap<ManiaHitObject>
+    {
+        /// <summary>
+        /// The definitions for each grouping in a <see cref="ManiaPlayfield"/>.
+        /// </summary>
+        public readonly List<GroupDefinition> Groups = new List<GroupDefinition>();
+
+        /// <summary>
+        /// Total number of columns represented by all groups in this <see cref="ManiaBeatmap"/>.
+        /// </summary>
+        public int TotalColumns => Groups.Sum(g => g.Columns);
+
+        /// <summary>
+        /// Creates a new <see cref="ManiaBeatmap"/>.
+        /// </summary>
+        /// <param name="initialGroup">The initial grouping of columns.</param>
+        public ManiaBeatmap(GroupDefinition initialGroup)
+        {
+            Groups.Add(initialGroup);
+        }
+    }
+}