Draft: mention sound bug fix (issue #1228)

[BennyyyBB]

Proposed changes

The user described a bug (issue #1228) that caused the mention/reply sound to not play and automatically reset in the settings. Having a look at the codes I figured out that the built-in mention/reply highlights are being edited through the custom-highlight UI even though they are intentionally non-persistent. Therefore I fixed two main things:

• built-in mentions/replies are no longer displayed as editable "Custom Highlights"
• clicking "Default Sound" directly resets the soundDef

Types of changes

• Bugfix (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)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged

Further comments

1. SInce this is just a draft for fixing the issue, there might still be errors or more improvements to me made.
2. Additionally we could think about integrating the feature that introduces different sounds for mentions and reply sounds. (Persistent settings like highlight.basic.mention_sound_path and highlight.basic.reply_sound_path would have to be added.)

If this fix was added to CHANGELOG-nightly.md it could be called: fixed an issue that caused mention/reply sounds to not play properly due to non-persisting sound settings