Allow modifications to source attachments of libraries

[elharo]

[akerekes]

Implement this interface for classpath attribute editing: https://github.com/eclipse/eclipse.jdt.ui/blob/master/org.eclipse.jdt.ui/ui/org/eclipse/jdt/ui/wizards/ClasspathAttributeConfiguration.java

This interface is for classpath container editing: https://github.com/eclipse/eclipse.jdt.ui/blob/master/org.eclipse.jdt.ui/ui/org/eclipse/jdt/ui/wizards/IClasspathContainerPage.java

[chanseokoh]

Why is it that we don't have source for the libraries we attach? It happens to App Engine API libraries too.

[akerekes]

Because the sources are not published to Maven or available as a separate download. They're available in the download but we haven't configured our libraries to use that for a few reasons: it's 200Mb, it's inside the zip and we don't yet support the sourceAttachmentRootPath attribute for the classpath entries we create.