New camera and entities

I replaced before camera what was setting rendered view by changing view
of render window to better alternative what is rendering only things
what are around player, and not moving them off screen like before
method did. Also integrated entities to screen manager, so to
draw/update/load entity you just add entity to entity list what is
stored for each screen. Entity list will decide if it can render, update
and load your entity based on IDrawable, ILoadable, IUpdateable and
Drawable (SFML) interfaces. Also fixed major bug with not clearing gui
manager after switching screen.