tmf: Modernize ITmfEventField

tmf: Modernize ITmfEventField

Remove the notion of field indexes. Fields should be accessed by
name, not by arbitrary numerical value. Replace all arrays in the
API by Collection<String>. (Iterable would have been better but
many places made use of .size(), didn't want to break too much
things).

However, TextTraceEventContent made heavy use of field indexes.
Left this behavior as-is, by simply returning a List instead and
making those methods specific to this implementation.

Also fixes bug #433801.

Change-Id: I889355f86a3141d92ff2af3f30afec13f05152dd
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/25761
Tested-by: Hudson CI