[Toolkit.Game] Fix issue #363. Perform a ClearState before a new BeginDraw to ensure that there is no unstable d3d11 device states. Toolkit samples should work on Nokia handset
[Toolkit.Graphics] Allow to specify the DepthFormat a constructor time with a RenderTargetGraphicsPresenter
[Toolkit.Graphics] Use readonly on fields that should not be modified
[Samples] Hide SystemTray on all WP8 toolkit samples