[FIXED JENKINS-21399] Job Properties should not be exposed for MultiBranchProject instances as MultiBranchProject does not inherit from Job...

A side effect is that we need a MultiBranchProperty that is distinct from the BranchProperty (i.e. JENKINS-22242)

A second side effect is that we need a concrete version of that class that implements the Project Based Access Control functionality (i.e. JENKINS-22243)