Issue: Stackable Page Types

For clarification, it's not possible to do exactly what was mentioned in the first post. Some node types are still distinctly different and can't be stacked in this way: Index, Area, Group, Book, Category, Attachment, Page.

However...
Each node type can have 1 Page as content. This page can take on the following page types: none, Template, Island, or Header.

Additionally,
Books, Categories, Attachments, and Pages can be added to Areas, Groups, Books, or Categories as children.
All these node types can receive their own comments without having to create Page-content first.

Re-opening. For backwards compatibility with some pages in VaultWiki 3, some node types do in fact need to stack:

- Area + Category/Book/Attach
- Index + Category/Book/Attach
- Group + Category/Book/Attach

To accomplish this, we need the changes made for Node History (all nodes are pages), and we need a container UI class (UI/Stack) that produces the output by selecting parts of other UI based on the item's types. To handle stacked permissions, we need a respective UI/Stack model that passes the item to each appropriate UI/Tab model and verify that all return true.

Closing this. The code is in place to do this, and the existing (working) page actions have been updated. I'm pleased with the changes; it's much easier to modify the page output than before. Some cleanup still required, as I suspect some classes are now unused or unnecessary mediators, but we'll worry about that later.

Originally suggested for 3.0.5.