Naming and inheritance changes of the top-level class holding active views and...

Naming and inheritance changes of the top-level class holding active views and managing container tree

- removed inheritance from a component
because it could incorrectly indicate
that this class is only a component
- changed name from RootComponent to
ViewManager (applies to the point above)
and since the introduction of the assertion
that only a view object can be displayed
and not a component, a view manager is more suitable
(This class could set the currently displayed view)