tmf: Replace Color resource with RGBA descriptor in marker events

This avoids the marker event sources from having to manage, create and
dispose color resources.

Also adds a RGBA to Color map in the time graph color scheme object
which is shared between the time graph control and scale.

Change-Id: I168c949e2aa63c0f5e958385490dc2543ecde7a5
Signed-off-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-on: https://git.eclipse.org/r/66046
Reviewed-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>