StackTrace.Entry: output containing resource name if possible
interpreter: fix stacktrace handling in combination with returns

If a ReturnEncounteredException is thrown, the current
StackTrace entry must still be removed from the stack.
This can not only happen in Blocks and funcs, but also
in anything that contains a block, e.g. loops and ifs.
Append the exception cause when printing an InterpreterException
StdlibUtil: create copies of parameter EObjects

This is a reoccurence of #1, where associating EObjects
with a new parent destroys the original associations.
add Tuple class to core
specify model classes in XCore
add XSemantics logic for tuples