tmf: Introduce a central trace manager

tmf: Introduce a central trace manager

This manager will track all currently opened traces and their
currently selected timestamps and time ranges.

This will also give both views and *.core analysis plugins
a centralized way to get the current active trace/experiment.
This was not an easy thing to do reliably if you were not an
instance of TmfView.

Basic unit tests are included, more complete tests will be added
later.

Change-Id: Ie0056252908ae8bb9e5da33242c3141665eb161c
Signed-off-by: Alexandre Montplaisir <alexmonthy@voxpopuli.im>
Reviewed-on: https://git.eclipse.org/r/12079
Tested-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
IP-Clean: Bernd Hufmann <bernd.hufmann@ericsson.com>