Began work on scene system
- Overhauled Entity class
- Removed IDrawable & IUpdateable
- Removed Window property due to limited use
- Added Scene class
- Changed sample to work with the scene system
Minor cleanup

- Fix some inconsistencies in the coding style
  - Change spaces to tabs in some places
  - Add a space before the braces in some if-statements
- Rename Void.Window.RootScene to Void.Window.Scene
- Make Void.Entity.All read-only
- Make the Void.Window.Scene setter public instead of private
- Add a call to base.Draw in Void.Samples.Windows.Background.Draw
Merge branch 'feature/scenes' into develop