Allow source folders to be configured for the individual slices of a 
multi-release jar. 
+ This is done using a new classpath attribute "release"

Enhance builder and reconciler such that sources in release-specific 
folders will be compiled with the correct matching context:
+ compile against the matching release of the JDK
+ compile against other classes of the same or lower releases

Includes a change of internal API, used in PDE API tools, which will
be compensated by https://github.com/eclipse-pde/eclipse.pde/pull/1805

Fixes https://github.com/eclipse-jdt/eclipse.jdt.core/issues/265