Platform: no need to have GlutApplication::redraw() virtual.

The other implementations aren't virtual too.
Platform: documentation updates.

Sdl2Application is now taken as base implementation (it was GLUT
previously) and all others are copying/referencing the documentation
from it. When SDL2 is included in all major distributions (Ubuntu, I'm
looking at you), it will replace GLUT as the default application.
Platform: cleaned up unneeded includes.

Also deinlined pure virtual function implementation.