Optional targetFolder & targetLibraryFolder and their default values

[jdicks]

During work on #499 we discussed that targetFolder & targetLibraryFolder might be optional in future.

If they are optional the default values currently defined might not be the optimal for all target languages. the default values are defined as

• targetFolder -> src-gen
• targetLibraryFolder -> src which is a typical java-based assumption. C developers might expect the sources will be generated within root directory for example.

Defining sufficient default values have to be dependant on the target language ( or the generator used).

if this is done the parameters (targetFolder, targetLibrary) might also be optional in future.