Replaced usage of swt-lock by Java's synchronized.

The swt-lock default implementation does nothing, this didn't help.
The extra class for synchronized interpreter access can be removed as soon as Xtend supports synchronized natively.
Added toString for easier debugging.
Replaced evaluation of rich strings.

We are using some Xtend infrastructure now in order to re-use the magic indentation from Xtend rich strings.
The two kinds of trace trees are still not unified (yet to come).