Initial implementation of the EditorTimingTimeline in TestCaseEditorTimingTimeline

Will probably rename this to MiniTimeline or something... But the basic structure is there plus most of functionality minus keyboard input.

osu.Game/Beatmaps/BeatmapInfo.cs

@@ -84,7 +84,7 @@ namespace osu.Game.Beatmaps
         [JsonIgnore]
         public int[] Bookmarks
         {
-            get { return StoredBookmarks.Split(',').Select(int.Parse).ToArray(); }
+            get { return StoredBookmarks?.Split(',').Select(int.Parse).ToArray() ?? new int[0]; }
             set { StoredBookmarks = string.Join(",", value); }
         }