Define new methods on Plugin for DI.

New dependency-injection supporting methods, and ample javadoc.

Stub the methods out in BasePlugin so everything carries on working.

Change-Id: Iaea88b091a4680c568f8e55a75a3b0d97c9842eb
Reviewed-on: http://gerrit.dmdirc.com/2846
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>