More interfaces, fixed timestep

So in this commit I overhauled use of Content Manager. Now it is only
used in LoadContent(ContentManager content) void to avoid confussion and
misuse of it. Also, move ILoadble to Spooker.Content and IUpdateable to
Spooker.Time to avoid confussions. Also, now you can bind Camera to
object what it should follow by inheriting interface IFollowable to
object and then setting camera.Follow to that object. Also, changed
InputAction bindings to events, so they now cannot be triggered from
outside randomly. At last I added some more functionality to Vector2 and
Rectangle classes. Hmm, I almost forgot. I also created my own
implementation of SFML font and text classes, so now text can be easily
drawn with SpriteBatch.