TMF: Add an abstract tree viewer class

TMF: Add an abstract tree viewer class

Provides default functionalities for all tree viewers, like how to set the
column data, basic classes to manage the content of the tree, provide easy
abstract method to fetch the data to fill the tree.

Change-Id: I0b19027f24f9e612304bb89f32a03f8c67d8edf7
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/22184
Tested-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
IP-Clean: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>