This particular test may serve as sample code in various materials in
the future, therefore it is highly beneficial if it is kept in good shape.
Make it easier to read, more compact, and split up among the natural
boundaries of the functionality (global setup in main, window+swapchain
management in Window, graphics resource setup and draw call recording
in HelloWindow).
Change-Id: I2451d3961a01131dcbffe66baf23d2cf9bfd077f
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>
