Added utility functionality to extract user specified classes to skip during code generation based on a user input string
Added some tests to exercise parsing of user input data concerning user specified classes to skip
Added support for user specified classes to skip during Java code generation
The Java code generator now allows the user to specify which classes to code generate.

This required changing the preference page quite a lot and handling of storing
and loading of preferences as well.

Close #382
Added error handling for when the Java code generator cannot write the generated Java source code files to disk

Issue #381
Slight improvements to the test results related to generation of the field expression
Added test revealing problem with generation of the field expression when the object type is a union type containing the unknown type
Fix for problem with generation of the field expression when the object type is a union type containing the unknown type
Merge remote-tracking branch 'origin/pvj/main' into ncb/development