Graphics: Finish GUI manager stub

Now GUI manager can be created. Also, now GUI manager owns IInputManager.
Graphics: Delete PlatformData & Move GUI manager back to hrengin::gui
Common: Delete typedefs of Vector2d

I really don't like that. Few more characters won't hurt, and I think
`Vector2d<i32>` is clearer then this: `Vector2di`.
Common: Add Rect class

Primarily used for defining GUI element sizes.
Graphics: Implement basic GUI manager functionality

o Wrapper for GUI Window
o Adding new elements