Features/middleware

[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 collect:
  • collect will return a dict where the extension_id will be used as key and the returnvalue as value.
  • 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.
  • We have some documentation here
• 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.

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[k9ert]

Closing this in favor of #2186
This should go into uiux-revamp and not into master.