[New] Implemented StrolchTransaction.findElement(Locator)

Now using a locator, it is possible to query a StrolchElement. It is
possible to query top level i.e. Resource/Order or ParameterBag, or
Parameter.
[Bugfix] fixed broken synchronisation in CachedElementMap

It is still not completely clear if this is correct - tests will follow
- but this is certainly better than before...