Polished PR that fixed deadlocking and crashing with items

[TimOliver]

• Renamed isEmpty to hasNoSections for better clarity.
• Fixed up infinite loop that occurred when setting items to nil.
• Fixed infinite loop when trying to generate separator views when items is nil.
• Made unit tests more comprehensive in initialising the class.
• Moved test code from CodeViewController to unit tests to capture fixed crashes.
• Added cancel logic when a segmented control lost focus mid-tap.
• Renamed addNewSegment to addSegment as new isn't necessary.
• Renamed most Swift APIs to match their counterparts in UISegmentedControl more.