Look for blocks properties/content

[alxlg]

I think it could be useful to look for properties and content of blocks too.

In particular I'm referring to:

1. the parent block if any (maybe fallback to current page title and properties otherwise?)
2. Previous/next sibling block

An use case for 1. could be:

You are in a page/block about an Author and you want to use a "New Book" template to create a Book that has page title/block content as the value of author:: property:

- Name Surname
  - Title of the book created with a template
    author:: Name Surname