Improve build definition handling

Build definitions are very awkard to use in heterogenous groups, such as Apache Commons.

If there are multiple Group definitions, it appears that projects cannot select any but the default group build.

It would be nice if alternate builds could be created (e.g. Java 1.5, Java 1.6) and then individidual projects could override the default

At present, the only way to override the group default is to create a project build. However, once created, it does not appear to be possible to disable it except by deleting it. Sometimes it would be useful to be able to use a different build, e.g. to run more thorough tests, or enable debug etc.