Nullability annotations and other assorted forms of Objective-C modernisation.

[mz2]

This PR contains the following modernisation changes:

• Adds nullability annotations across the API.
• Module style imports.
• Makes the timer's object automatically nullable (instead of unsafe, unretained).
• Hides ivar declarations in the implementation.
• Drops some stuff that's no longer relevant with current SDKs.

You may also be interested of MPTimer – a Swift port of MABGTimer which I made just now to allow using a pure Swift object as the object with the timer.