Eve::Eng::Input added. Eve::Eng::Graphics::Window changed to Eve::Eng::Window. Eve::Environment created.

Created files for Eve::Eng::Input and base class, excluding system
events management (i.e. callback system) and excluding basic input /
event system.
Moved the Eve::Eng::Window class from the Graphics namespace, moved the
graphical context class inside it (as it is part of the display
toolchain).
Started writing the Eve::Eng::Environment class with singleton pattern.