Implement stack based state system

[soulweaver91]

Rather than having all menu functionality in one class, have each unique screen as its own class and implement the menu system as a stack of those screen classes.

The level manager could potentially also be put on the stack, so a pause menu could work in top of it easily.

[soulweaver91]

Some drafting for new APIs to implement:

// menus and LevelManager descend from this class
class EngineState {
public:
    EngineState(T params);
    ~EngineState();
    // broadcasted to all states; passive states can still draw if necessary
    // (for example: have a separate state to deal with the background animations 
    // on main menu, submenus don't have to care about that)
    virtual void drawUpdate();
    // broadcast to all states? or just the topmost one? TBD
    virtual void handleInput(const QVector<T>& inputs);
    // Logic happens here. Should input go here as well?
    virtual void tick();
    void setActive(bool active);
    bool getActive();

private:
    bool isStateActive;
}

class CarrotQt5 {
public:
    void moveToState(EngineState* state, bool replace, T... params);
    // return new top state so that functions on it can be called if necessary
    EngineState* exitTopState(T returnValue);
    // stack cannot be empty, so a replacement bottom state must be provided
    void exitAllStates(EngineState* newRootState);

private:
    QStack<EngineState*> states;
}