Only allow one method of moving the player at a time

[valkyrienyanko]

Before you would be able to combine both keyboard and mouse movements to move twice as fast. This PR restricts this so only one method may be used at a time. That is keyboard or mouse but not both.

Before

Velocity =
    new Vector2(
        Input.GetActionStrength("ui_right") - Input.GetActionStrength("ui_left"),
        Input.GetActionStrength("ui_down") - Input.GetActionStrength("ui_up")
    ).Normalized() * Speed;

if (Input.IsActionPressed("mousedown"))
{
    var direction = (mouse / main.camera.Zoom) - Position;
    if (direction.Length() > 1f)
        Velocity += direction.Normalized() * Speed;
}

After

if (Input.IsActionPressed("mousedown"))
{
    // Move player by mouse
    var direction = (mouse / main.camera.Zoom) - Position;
    if (direction.Length() > 1f)
        Velocity = direction.Normalized() * Speed;
}
else
{
    // Move player by keyboard or controller
    Velocity = new Vector2(
        Input.GetActionStrength("ui_right") - Input.GetActionStrength("ui_left"),
        Input.GetActionStrength("ui_down") - Input.GetActionStrength("ui_up")
    ).Normalized() * Speed;
}