[INTERPRETER]
* NEW: Check special methods return value datatype and raise a dedicated error if it is incorrect.
* NEW: Error.Text, Error.Where and Error.Backtrace are now available inside the Application_Error handler.
* BUG: 'gbx3 -L' now displays the same Gambas version as 'gbx3 -V'.
* NEW: A class can declare a public '_invalid' boolean variable. The value of that variable will indicates if the object is invalid.