tmf: Move some code to copy/add/remove traces in UI

tmf: Move some code to copy/add/remove traces in UI

The "Copy trace", "Select traces..." and "Remove" functionnalities have
some code that can be reused by each other and future features.  This
code has been moved to the model element's classes.

Change-Id: Id61914521f41d9c2b2c25fe1268e61be62c1d504
Signed-off-by: Geneviève Bastien <gbastien+lttng@versatic.net>
Reviewed-on: https://git.eclipse.org/r/10751
Tested-by: Hudson CI
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>
IP-Clean: Patrick Tasse <patrick.tasse@gmail.com>
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>