Adds a mechanism through which windowing clients can re-request an
UpdateSystemTheme message. This is currently used in SystemMenu's
ShutdownDialog to refresh it's theme when the dialog is instantiated.
Instead of storing the back/front buffers of a GUI::Window as just
Gfx::Bitmap, wrap them in a WindowBackingStore class.
This will allow us to add more information alongside the bitmaps while
keeping the back/front swapping logic simple.
