Updates to SANTab internals: encapsulated tab contents into a private custom view subclass, SANTabContents.  This will facilitate hiding/showing contents when the tab is selected.
Collapsed method “drawTab” into “drawRect”.
- Renamed Tab-related classes;
- Cleaned up internals of TabView;
- Drawing arcs at the bottom of each tab header;
- Added NSView+Utils category, primarily to help setting constraints between subviews and superview.