ISIS-618: standalone collections now rendered in own page...

... rather than inlined as part of the action page.

Also:
- renamed ActionPage to ActionPromptPage (because that is what it's responsibilities will ultimately be)
ISIS-618: value and void returns now also delegate to separate pages

... ie VoidReturnPage and ValuePage.

ActionPage renamed to ActoinPromptPage.

In addition, the logic in StandaloneValuePanel, handling blobs vs clobs vs urls,
is now moved earlier into ActionPromptPage.

Also, got rid of ActionPage.SingleResultMode, since no need to render results inline.

Also, removed (partially implemented) support for the rendering of transient objects;
will fail fast.  (Previously Wicket viewer would render the object, but didn't
provide a save button.  So clearly this feature isn't being used).

Also: added some hackery to BookmarkPagesModel, to protect against earlier failures
corrupting the rootNodeList (preventing any subsequent ability to use the app).