Add a $adjustDepth parameter to CategoryModel::GetSubtree().

This makes it easier to provide a subtree of categories to modules and views that looks exactly like the full tree of categories.
Refactor the CategoriesModule.

- Use the standard Gdn_Module->View property instead of using custom module hiding logic.
- Get the data for the module only on render, not initialization.
- Throw an event so that plugins can change the data in the module.