(see Desc) Added Utility::centerOrigin()

* Added Utility.h/cpp
Utility.h/cpp implements centerOrigin(), which will take either a
sf::Text or a sf::Sprite and set its origin to its center.
(see Desc) Almost Finished States!

* The game is now run via Application::Run(), rather than Galaga::Run()
* New EntityDragger() sets the velocity rather than accelerating it
* Added Title Screen, Main Menu and Pause Screen
* Added placeholder options for the Scoreboard, Local Multiplayer and
Online Multiplayer screens
* Multiple states at once don't quite work as intended... some bug in
the StateStack somewhere. It SEEMS that it's drawing through and
updating through the stack in the same order, but drawing through the
stack in reverse-order doesn't change the render order, nor does it fix
the issue. I need to do more research (or take a breather?)