Allowing the player to level up and spawning a new wave

In the following code, we allow the player to level up between waves. Because of the work we have already done, this is straightforward to achieve.

Add the following highlighted code to the LEVELING_UP state where we handle player input:

// Handle the LEVELING up state
if (state == State::LEVELING_UP)
{
    // Handle the player LEVELING up
    if (event.key.code == Keyboard::Num1)
    {
        // Increase fire rate
        fireRate++;
        state = State::PLAYING;
    }
    if (event.key.code == Keyboard::Num2)
    {
        // Increase clip size
        clipSize += clipSize;
        state = State::PLAYING;
    }
    if (event.key.code == Keyboard::Num3)
    {
        // Increase health
        player.upgradeHealth();
        state = State::PLAYING;
    }
    if (event.key.code == Keyboard::Num4)
    {
        // Increase speed
        player.upgradeSpeed();
        state = State::PLAYING;
    }
    if (event.key.code...