TMF: Changed the state system explorer to use an abstract tree viewer

TMF: Changed the state system explorer to use an abstract tree viewer

This adds the possibility to sort columns. And we don't need to worry about
which thread runs the code.

Change-Id: Iee53523f05eb3a3bdc6bf4d4226cc22a93bf9864
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/22185
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
IP-Clean: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Hudson CI