Extension callbacks refactoring and middleware implementation

[k9ert]

The current implementation of callbacks has a major flaw: The callback can collect results from different extensions but it can't let extensions modify the results of former extension-modifications.

So this PR does those things:

• Refactoring of the ServiceManager to an ExtensionManager including references from services to extensions
• Redefining the callbacks in callback.py to enable different return_styles. The string representations has been modified to be using classes
• Implementing a new return_style called "middleware" and the old-style will be called dict:
  • dict will return a dict where the extension_id will be used as key and the returnvalue as a value (to be renamed to "collect")
  • middleware will assume that something will be passed to the callback as value and the returnvalue (might be the same or different) will be passed to the next callback of the next extension until the last extension's returnvalue will be returned by the execute_ext_callback-method.
• The extensions might need order for that. So now every extension can specify a depends Attribute with a list of Extension-classes. The callbacks will get called so that the dependency tree and the sort-priority is respected.
• Also we're implementing here a way for extensions to create their own callback-classes.
  

On top of those changes, a new graphql-extension has been created:

• Will expose a /graphql endpoint
• Creates a new callback of type middleware which starts as an empty array and the extensions can add fields to it.
• As an example, the devhelp extension adds a field called "bookmarks" and you can do a query (on /graphql) like:

{
  bookmarks {
    title
    desc    
    
  }
}

• Additionally, the extension is suppose to expose core-types. As an example, the users are exposed. Query like:

{
  users {
    username
    
  }
}

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[relativisticelectron]

When you refactor the locations of the services, can you also solve #1935 ?

[k9ert]

This has been closed and instead we have now #2105 and #2106