Hierarchical facet ui

Some more work on the way to hierarchical facets.

The PR get the new stuff at a point where it would be possible for someone to start working on the UI, look and feel a bit more.

I intend to continue working on geonetwork/search interaction.

The old, non-hierarchical way of doing things should still work so long as the feature toggle in `Config.groovy` remains off - but I invite the reviewer to double check this.