maven plugin need to populate group-level metadata.xml 
with plugin prefix

JENKINS-14325: CCE & missing GUI when using jobs inside folders

Conflicts:
	changelog.html

[FIXED JENKINS-14365] Show a ChangeLogSet.Entry.revision as a <li> value only if it is in fact an Integer.

JENKINS-14330: NPE from UnlabeldLoadStatistics.computeTotalExecutors

and item renaming in relative item names

Introduced AbstractProject.supportsMakeDisabled rather than using trick of checking for parent instanceof Jenkins.
Extracted common GUI into makeDisabled.jelly, with accompanying I18N refactoring.
Also moved all this into AbstractProject, where the relevant methods are defined, rather than Job where they are not.

Fix NPE during loading matrix job
Thanks!