Normally if a user want a custom crop tool bar, the tool bar need to delegate operations to CropViewController. For some users who do not want the attached tool bar but still want to use these delegate functions, there is no need to pass a CropToolbarProtocol instance. So I change it to optional
Normally if a user want a custom crop tool bar, the tool bar need to delegate operations to CropViewController. For some users who do not want the attached tool bar but still want to use these delegate functions, there is no need to pass a CropToolbarProtocol instance. So I change it to optional