Create a basic architecture for screens and screen transitioning

[silas-hw]

Make the damn architecture for screens!

Easiest approach will likely involve using a sequence action with Scene2D.

Current idea:

• make a simple scene2d actor just holding an alpha value
• add a switchScreen method to screens, starting a new sequence action to fade out and then set the screen
• screens can do the opposite when the show method is called to fade in.

[silas-hw]

Similarly, I could override the setScreen method and do it all in there. That would arguably be less gnarly.

[silas-hw]

Ended up creating a record to pass common screen arguments around as well

[silas-hw]

Screen transitions are in B) can also mess around with a wait time to force some blank frames before starting the next screen.

Transitions happen in the GameScreen itself, not the Game class. This allows the GameScreen to freeze physics updates whilst transitioning! Awesome!