[FIXED JENKINS-9913] At least print a diagnostic to the build log if we are waiting on a checkpoint.

BuildStepCompatibilityLayer already implements BuildStep, so its subclasses need not say they do as well.

Also [JENKINS-19042] does not seem to have made it into 1.526; seems to have been towards 1.527.

Follow-up on PR #848

The contents of those files are manually inspected.
This will hopefully reduce the ramp-up curve for new users.
This is based on http://www.jetbrains.com/idea/webhelp/project.html

This reverts commit 1a6f3dfa.
forgot to add this file

MAVEN_OPTS=-Dfile.encoding=Cp1252 mvn -DskipTests -f core/pom.xml clean compile

a search context is now provided to build SuggestedItem in order to
provide correct search name and url in the given results.

Integrated bytecode-compatibility-transformer that allows core to
do signature changes on properties that plugins might depend on.

The library performs necessary bytecode transformation to achieve this.

The first use of this is to fix plugins that looks for List
AbstractProject.triggers, thereby resolving JENKINS-18677.

For the time being, I'm not loading such compatibility annotations from
plugins, but I did code that in PluginManager. Let's see how this
feature work out for a while in the core, and if it looks stable and
solid, we'll open it up to plugins at that point.

Improve search to look for items inside ItemGroup

Added a new overloaded version that works on a project.

Updated CoreEnvironmentContributor accordingly.