scene: Split `graphics` into `scene' and `graphics'

I decided to rework some of engine's architecture, since it was
completely pointless to have two separate hierarchies (namely
IBaseEntity with entity parentsystem and ISceneNode with underlying
scene manager's hierarchy). Now I am reworking it all to have a single
solid hierarchy system.
game: Remove parentsystem of IBaseEntity & rename class to IEntity

See previous commit.
graphics: Move scenegraph-related classes to namespace `scene'

See bb38b019d18f372046ef240b2e7d60029d8e97b1
game: Add CEventManager destructor

Probably something related to future event system rewrite
common: Fix compile errors
graphics: Explicitly list includes in hrToIrr.h
io: Add virtual destructor to ICharacterStream

Memory leaks, yo