This changes the editor to track the current track as it is *loaded* by
`MusicController`, rather than haphazardly following the current global
`WorkingBeatmap` (with a potentially unloaded track) or relying on local
immediate-load behaviour (as implemented in `ResourcesSection`).
Not only does this combine the check into one location, but it also adds
a check on the global `WorkingBeatmap` being updated, which is the only
way I can see the following failure happening:
```csharp
05:19:07 osu.Game.Tests: osu.Game.Tests.Visual.Editing.TestSceneEditorBeatmapCreation.TestAddAudioTrack
05:19:07 Failed TestAddAudioTrack [161 ms]
05:19:07 Error Message:
05:19:07 TearDown : System.NullReferenceException : Object reference not set to an instance of an object.
05:19:07 Stack Trace:
05:19:07 --TearDown
05:19:07 at osu.Game.Database.ModelManager`1.<>c__DisplayClass7_0.<AddFile>b__0(TModel managed) in C:\BuildAgent\work\ecd860037212ac52\osu.Game\Database\ModelManager.cs:line 36
05:19:07 at osu.Game.Database.ModelManager`1.<>c__DisplayClass8_0.<performFileOperation>b__0(Realm realm) in C:\BuildAgent\work\ecd860037212ac52\osu.Game\Database\ModelManager.cs:line 49
05:19:07 at osu.Game.Database.RealmExtensions.Write(Realm realm, Action`1 function) in C:\BuildAgent\work\ecd860037212ac52\osu.Game\Database\RealmExtensions.cs:line 14
05:19:07 at osu.Game.Database.ModelManager`1.performFileOperation(TModel item, Action`1 operation) in C:\BuildAgent\work\ecd860037212ac52\osu.Game\Database\ModelManager.cs:line 46
05:19:07 at osu.Game.Database.ModelManager`1.AddFile(TModel item, Stream contents, String filename) in C:\BuildAgent\work\ecd860037212ac52\osu.Game\Database\ModelManager.cs:line 36
05:19:07 at osu.Game.Screens.Edit.Setup.ResourcesSection.ChangeAudioTrack(FileInfo source) in C:\BuildAgent\work\ecd860037212ac52\osu.Game\Screens\Edit\Setup\ResourcesSection.cs:line 115
05:19:07 at osu.Game.Tests.Visual.Editing.TestSceneEditorBeatmapCreation.<TestAddAudioTrack>b__13_0() in C:\BuildAgent\work\ecd860037212ac52\osu.Game.Tests\Visual\Editing\TestSceneEditorBeatmapCreation.cs:line 101
05:19:07 at osu.Framework.Testing.Drawables.Steps.AssertButton.checkAssert()
05:19:07 at osu.Framework.Testing.Drawables.Steps.StepButton.PerformStep(Boolean userTriggered)
05:19:07 at osu.Framework.Testing.TestScene.runNextStep(Action onCompletion, Action`1 onError, Func`2 stopCondition)
```
