This patch introduces a Tree component, implemented as a Composite
using TreeGrid. As initial version of the component, some functionality
of the old Tree component has been left out for now.
Partly addresses #8617
Move and rename server classes which go into the compatibility package
* Use com.vaadin.v7
* Use the same class name as in Vaadin 7
* Use a "vaadin7-" declarative prefix for Vaadin 7 components
Change-Id: I19a27f3835b18980b91a4f8f9464b2adde1a5fd5