Row classes cannot be subclassed when installing with the Swift Package Manager

[RobertAudi]

When trying to subclass a row class Xcode throws an error:

class CustomRow: BaseRow { /* */ }
// Error: Cannot inherit from non-open class 'BaseRow' outside of its defining module

This is because the row classes are defined as public instead of open.

The problem shouldn't exist when the source files are dragged into a project though.

[haojianzong]

Thanks for bringing this up @RobertAudi

I will update.

[haojianzong]

15 addresses subclassing. I've made BaseRow and FormInputCell open and keep other rows and cells public because I think inheritance will make the code harder to maintain both for this repo and the caller application. Please check the readme.md change here.