Minor simplifications to TabView: removed unused code.
Better organizing classes used for rendering tabs:
- created a “tab” view subfolder;
- moved classes MultiTabView and TabView to view/tab subfolder.
Removed stale references to MultiTabView & TabView from Xcode project.
Simplifying TabView implementation, as it has grown quite a bit: extracted private classes TabViewHeader and TabViewContents as stand-alone classes.