Add reusable Button component

[sanderfrenken]

I could make a start where you can init a button which you need to provide a texture for normalState, a texture for pressedState (optional), an action (optional), a font (optional) and a text (optional) and optional actions to run on touchesStarted/ touchesEnded. That way, you can give an action like `fadeOut` or `scaleTo` so it also facilitates some more complex stuff.