Runtime switching of experimental mode

[andybak]

Allows switching experimental brushes on and off at runtime. This fixes a couple of issues:

1. Errors loading sketches with experimental brushes in them
2. Issues with multiplayer where experimental brush strokes wouldn't appears if a user had experimental brushes disabled.

Now - the mode switch only affects the brush panel UI. All brushes from both manifests are always loaded.

This also cleans up and refactors some stuff that remained from the experimental mode removal (brush remapping is active all the time as it doesn't do anything unless you explicitly use it)

After merging - one change will be needed to the multiplayer branch. In MultiplayerStrokeSerialization.cs line 97:

foreach (BrushDescriptor brush in App.Instance.m_Manifest.UniqueBrushes())

becomes

foreach (BrushDescriptor brush in App.Instance.FullManifest.UniqueBrushes())