Implement the screen update routine emulation.

The routines are:
- SDL_Flip
- SDL_UpdateRect
- SDL_UpdateRects
Add the initial part of the keyboard emulation.
Add some keyboard compatibility constants.

They are compied from SDL_compat.h.
Add emulation for several keyboard functions.

The following functions are a pure NOP in SDL 2.0:
- SDL_SetKeyRepeat.
- SDL_GetKeyRepeat.

The SDL_EnableUNICODE is an emulation function, whose code is based on
the code in SDL_compat.
Improve some doxygen comment.