Simplify resource values serialization

Serialization of resource values (values stored in a separate resource file) currently is using several pre and post processing steps and logic spread across ModuleWriter, ModuleContext and SourceSerializer implementation(s).
This complicates further/future maintainability and extendability while the logic and requirements (now) have been fleshed out enough to allow refactoring this in a more straightforward way.

To simplify the resource values serialization the following changes are needed:

• serialize resources during serialization of a source instead of afterwards via a PostProcessItem (CopyItem or BinaryItem)
• move the responsibility for serialization to the ModuleContext (to be invoked from the SourceSerializer)
• ModuleWriter and SourceSerializer will explicitly (and thus only) use model implementation classes like ModuleImpl, DefinitionNodeImpl, DefinitionPropertyImpl, ValueImpl, etc. to no longer need to use awkward casts where it matters