TMF: Add possibility to select experiment type from UI

TMF: Add possibility to select experiment type from UI

Modified trace type contribution item to element contribution item to add
experiment types to the list and show only trace or experiment type depending
on the selected element. Add the menu items for the experiment type selection.

Change-Id: Id566ce7c30a89d5371c2c311d7d96c1f065bf7fa
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/20669
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>