Allow management of project building request kept inside MavenProject instances

For lazy construction/loading of some bits, MavenProject instances currently hold a reference to the ProjectBuildingRequest from which they were created. This proves to be problematic for IDE integration where those requests
a) hold a fair amount of memory (in particular the repo system cache)
b) hold references to potentially obsolete objects like GUI-backed transfer listeners

As such IDE integrators have the need to either nullify or update this request.