Support for placeholders in a configuration that represent a pluggable
UI block, i.e the blocks used for downloading extensions, managing
plugins, and managing languages.
A pluggable block may be a simple section with dynamically generated
configuration entries (e.g. generated programatically after inspecting
the disk) and/or may have special semantics, or may have a special UI which
may override the behaviors of a standard item. Normally pluggables
are sections, containing values and other sections. If this isn't
appropriate, you will have to implement a new visit() function for
saving configuration data.