[Refactor] Use existing method as shortcut

Instead of re-implementing the same call chain
[Refactor] Renamed TargetInstaller#target to native_target
[Refactor] Renamed Target#target to native_target

This should make it more intuitive why we have to navigate there.
[Refactor] Renamed local var in spec
[Refactor] Renamed TargetInstaller#library to target
[Refactor] Extracted Generator::Header from PrefixHeader

Fixed wrong documented types drive-by.
Minor change for edge case, more intuitive empty line generating
[Doc] Fix attribute documentation

Thanks to @fabiopelosin.
[Changelog] Add entry for #2461
Merge pull request #2461 from mrackwitz/refactorings

Refactorings in Preparation to Framework Support