[ui] Add base classes for mobile applications

ViewManager, View and ViewModel provides the basic infrastructure for
implementing applications which consist of multiple view hierarchies.
[locale] Add tart.locale for localization
[ui] Add TabBarView

TabBarView is a composite tart.ui.View which includes a tab bar and
many other views associated with it. It can automatically set views
based on a user's interaction with the tab bar.
[components] Add infinite scroll component

This component is useful for loading content into a view based on its
scroll position, i.e., when a user scrolls to the bottom.
[components] Add P2RComponent for pull to refresh

This component is useful for refreshing a view's content based on its
negative scroll position.
[components] Add SidebarComponent

This component is useful for implementing view hierarchies that makes
use of a hidden sidebar, which is revealed through a panning motion.
[components] Add NavBarComponent

This component offers a basic navigation bar for other views. It
includes back and menu buttons. Still in early development.