Reoganised package structure of language-base project
Reorganised package structure of language-jvm project
Moved a few of the new 'legacy' plugin classes into the 'plugins' project
Update docs for renames
Spec work
Implemented a subtyped view over a PolymorphicNamedDomainObjectContainer
Added 'nativeLibraries' container as typed view over 'libraries'

- Removed support for creating untyped elements via 'libraries'
Renamed 'executables' container to 'nativeExecutables' in preparation for making it a filtered view
Updated samples for renamed containers
Integration test fixes