tmf: Enhance TmfEventField.toString() to also print sub-fields

tmf: Enhance TmfEventField.toString() to also print sub-fields

Move the logic to print sub-fields of a TmfEventField into the
base class' toString() method. This removes the need for a
specific CtfTmfContent type.

Change-Id: I4916ab9e0de843b2713ebc40ac8ad376848ab2c1
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/7382
Tested-by: Hudson CI
Reviewed-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
IP-Clean: Matthew Khouzam <matthew.khouzam@ericsson.com>
Tested-by: Matthew Khouzam <matthew.khouzam@ericsson.com>