Initial contribution of thing concept.

Bug: https://bugs.eclipse.org/bugs/show_bug.cgi?id=433994
Also-by: Oliver Libutzki <o.libutzki@external.telekom.de>
Also-by: Kai Kreuzer <k.kreuzer@telekom.de>
Also-by: Michael Grammling <m.grammling@external.telekom.de>
Signed-off-by: Dennis Nobel <d.nobel@external.telekom.de>
Could this be possibly moved to an internal package? The only class using it seems to be ThingManager, an internal class. If the ThingRegistry would move into the internal package (and we introduce a public interface for it), the addThingTracker method could be made package private.