(cherry picked from commit 9226b29d)

eagerly.

better to define a corresponding descriptor type for backward compatibility and methods like display name.

Add new extension point (class hudson.matrix.MatrixConfigurationSorter) for enbling to sort configurations of matrix job.

It would be better to find out why the nextBuildNumber is at times empty.
In the mean time this will find the appropriate number and write it to the file.

getBuildingUpstream() to public so they can be accessed from plugins,
small refactoring (reuse getTransitiveDownstreamProjects() and
getTransitiveUpstreamProjects() respectively)