Editor: Add feature to zoom to the current mouse position

I think this helps to improve the mapping workflow as you don't have to reposition the camera so much with this feature if you are used to it.

This code duplication can be avoided:

    // Determines in which direction to zoom.
    int Zoom = 0;
    if(Input()->KeyPresses(KEY_MOUSE_WHEEL_UP))
        Zoom--;
    if(Input()->KeyPresses(KEY_MOUSE_WHEEL_DOWN))
        Zoom++;

    if(Zoom != 0)
    {
        int OldLevel = m_ZoomLevel;
        m_ZoomLevel = clamp(m_ZoomLevel + Zoom * 20, 50, 2000);
        if(g_Config.m_EdZoomTarget)
            ZoomMouseTarget((float)m_ZoomLevel / OldLevel);
    }