Set up multi-platform generic Window and Graphics objects

[hnkb]

• Windows basic window implementation (mostly refactor available code)
• X11 basic window implementation
• Provide generic interface for both, probably with a pimpl pattern?
• Provide rudimentary platform-agnostic basic text rendering for bootstrapping
• Basic facilities for connection to graphics contexts
• Generic event system, for keyboard and mouse events
• Rendering to bitmap is useful, also for unit tests

What about window*impl?

This approach also limits following options

1. onMouse, onKeyboard, ... as virtual members that you inherit and customize
   This means you cannot do a js style

    Window w;
    w.onclick = lambda;

2. onMouse, kbd, ... As vector of lambda
3. single eventproc with minimal translation of messages
   This enables a camera object with sophisticated key and mouse events to be easily added

I shouldn't make this decision in a vacuum. I need a reasonably complex project in both x11 and windows and see what is actually needed and works best.

One related thing is: if I have multiple boxes (not necessarily windows), for example two textboxes, I probably do not want to create a class for handling each, and just provide a lambda for each of their event handlers.