Added Gradian support

Added Gradians to SpriteRotation (and improved documentation of
properties)
Changed constructor

Added AngleType and change SpriteRotation(float, bool) to
SpriteRotation(float, AngleType)
Added TextBoxSprite support for WhitePixel2D

Added constructor of TextBoxSprite that accepts no texture and uses a
WhitePixel2D.
Added remarks to TextBoxSprite.Moved