Support folders.

Needed to make this plugin work sensibly with the CloudBees Folders plugin, or generally with any item/view groups.

For example, create a subsubfolder with a couple of jobs in it; create a sectioned view on the folder; add a Job Graphs section. Before this PR you will only see top-level jobs/folders. Now you will see the jobs in the folder.

I may well have missed some less obvious places where it is assumed that things are at top level, but I fixed everything I found.