Restrict access to package-private vars in PhysicsComponent

[AlmasB]

File: https://github.com/AlmasB/FXGL/blob/dev/fxgl-entity/src/main/java/com/almasb/fxgl/physics/PhysicsComponent.java

These:

    FixtureDef fixtureDef = new FixtureDef();
    BodyDef bodyDef = new BodyDef();

    Body body;

should be private with appropriate accessors (probably package-private methods). Plus there is a getBody() method already, which can be used.