Further refactor/rewrite ModelBackedDrawable

[smoogipoo]

Attempt #2, since there were regressions relating to the placeholder.

Important changes:

1. Placeholder is now created via the CreatePlaceholder() method. It is null by default, not CreateDrawable(null).
2. ReplaceDrawable() has been split into ApplyHideTransforms() and ApplyShowTransforms().
3. The placeholder is shown in the background before a foreground model is hidden, and hidden in the background after a foreground has been shown.
   1. Naturally, this means there is now only one single placeholder, rather than created with every model change.
4. The placeholder is now in a DLW itself.
5. CreateDelayedLoadWrapper() now handles recreation of content internally, removing the need for code such as https://github.com/ppy/osu/blob/master/osu.Game/Beatmaps/Drawables/UpdateableBeatmapBackgroundSprite.cs#L39 in the case of using an unload wrapper.

[smoogipoo]

Superseded by #2467