Selections: Add support to make a value list available to the rendering code

On onehippo.com, we have a use-case where we want to make a value list available to the rendering template. The list is used to render front-end filtering options: the page displays an overview of documents that have a multi-selection field of our value list. By clicking on a certain filter link, the front-end would then hide all documents that do not have the corresponding value selected.

We added a rather generic HST component (org.onehippo.connect.components.ValueListComponent)
to the project, which takes a value list name (as defined in the project's spring configuration) as parameter, and makes the corresponding value list available as a request attribute. This component is rather generic, and only tied to the fact that we're dealing with a value list from the selections plug-in. As such, I think the plug-in should provide this HST component in a reusable manner.

Improvement: compared to the current project version, it may be nice if you could specify a CSV list of value list names, such that the component makes all value lists available as request attributes. It would also be nice if the plug-in could provide a static API for retrieving a value list, such that other components can reuse that code (Maybe part of the SelectionUtil class?).