title: Converting from Windows GDI to Qt
body: <p>I am evaluating the feasibility of converting my app (Win32) to Qt. I would like to convert the code so that I can gradually convert one single codebase from Windows GDI to Qt, rather than cloning the source and have duplicate code. By flipping one global switch, or declaring one manifest constant in the project properties, I want to convert and test, while being able to release a production version (with the old code) whenever necessary.</p>
  
  <p>Windows starts with a <code>WinMain()</code> function, I see that Qt has a <code>main()</code>.</p>
  
  <p>Is there an technique to organize the source to facilitate this? What is the best way to structure the code?</p>