Shaves >11min off the test time for maven-plugin on my machine!

See https://github.com/jenkinsci/maven-plugin

- set findbugs version to 2.5.2 to avoid:
[WARNING] 'reporting.plugins.plugin.version' for
org.codehaus.mojo:findbugs-maven-plugin is missing. ...

- delete maven-pmd-plugin and maven-checkstyle-plugin (commented out
since more than two years)

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

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.