Tab completion added to view magic which is now both a line and cell magic
Fixed bug in implementation of __add__ operator for GridLayouts
Label parameter of View objects has been declared constant
Views now have a style property (string) instead of a style parameter (dict)

Instead of individual View object owning separate dictionaries, style is
a property that operates on a string identifier. This makes it simpler
to manage style information across Views and Stacks and allows style
information to be easily shared.