Remove a dependency cycle involving StreamPlayer, StreamPlayerEvent and StreamPlayerEventLauncher.

[HelgeStenstrom]

Plus fixed a spelling error.

The cycle is removed by

• Make use of StreamPlayerInterface instead of StreamPlayer in StreamPlayerEvent
• Stop sending the source as a part of the event. The source was one link in the cycle.

Description

Please include a summary of the change and which issue is fixed. List any dependencies that are required for this change.

What kind of change does this PR introduce? (check at least one)

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Code style update
• [ *] Refactor
• [ ] Build-related changes
• [ ] This change requires a documentation update
• [ ] Other, please describe:

Does this PR introduce a breaking change? (check one)

• [ ] Yes
• [ *] No

Has This Been Tested?

• [ *] Yes, a little
• [ ] No

Checklist:

• [X] My code follows the style guidelines of this project
• [X] I have performed a self-review of my own code
• [X] I have commented my code, particularly in hard-to-understand areas
• [X] I have made corresponding changes to the documentation
• [X] My changes generate no new warnings
• [X] I have added tests that prove my fix is effective or that my feature works
• [X] New and existing unit tests pass locally with my changes
• [X] Any dependent changes have been merged and published in downstream modules