Multi-window & display support & resizing
- Multiple window support enhanced
- There can be several AppWindow instances.
- No default AppWindow instance is created anymore by Trieng or DemoApp.
  - User can decide when and how.
  - You don't need to decide the window size at creation of TriengApp/DemoApp => Constructor interface change
- Camera is now a feature of AppWindow
- AppWindows can now support resizing
  - Image2d supports aspect ratios better (window aspect ratio taken account)