tmf: Add element to its parent's children automatically and remove setChildren

Also, add a few useful setters.

Change-Id: I8ae7cff34d9a066d0ce9d17ef555e5539a3dc55c
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/35916
Tested-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>