Stream deletion propagated to Widgets

[janheise]

Description

The deletion event for a stream is read from the EventBus by a listener that checks all widgets for the stream reference and deletes the reference if necessary.

Motivation and Context

If a stream get's deleted, a widget might still reference that stream. This only becomes a problem (afaik) when the widget is part of a ContentPack that get's exported because during import, the stream can not be found and the import fails.

Because of possible performance issues, this is only a draft for now that might be picked up later after further discussion.

How Has This Been Tested?

manual tests

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• [] New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• [] My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.