ISIS-611, ISIS-612: disabled and hidden facets dynamically.

In addition, updated the LayoutMetadataReaderForJson to correctly export
json for these two tickets and also for ISIS-601, ISIS-602,
ISIS-603, ISIS-604, ISIS-605, ISIS-606, ISIS-607.
ISIS-612: URL as value type; returning URL opens in Wicket
ISIS-612: suppress veil for object actions.

In addition:
- some minor tidy-up of misc classes (no functional changes).