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
Revert "Tree item fails with ItemClickListener (#14388)"
This reverts commit d71ae18e35
which caused regression on #6845.
Converted TreeItemClickListeningTest (#6845) to TB4.
Change-Id: Ia4c7c5814dc4e8916ab3f80633ceecdfb74040b0