[!!!][FEATURE] Properly decode expression values

The expression value is now wrapped in an object to ensure correct
handling of the different data types. Decoding the XML response is
delegated to the expression value factory, therefore some tests are also
moved there (and the decoding is completed for the scalar value types;
objects will follow.)
[FEATURE] Map returned objects

Returned objects (including their properties) are now mapped to an
instance of an ExpressionValue subclass, "Object".
[FEATURE] Evaluate expressions in debugger engine

This implements support for DBGp’s "eval" command. The command class
has to be named "Evaluate", as apparently the Composer autoloader does
not support underscores in _ (otherwise I’d have named it Eval_, as the
PhpParser package does it; this however resulted in "class not found"
errors in the tests).