frontend: Update previewed sources in mixer by visibility

[Warchamp7]

Description

Improves the status of previewed sources in the mixer to be based on and update by scene item visibility.

Motivation and Context

Currently any source in the previewed scene gets flagged as Preview even if it's hidden and wouldn't be audible after transition. This change takes scene item visibility into account, and also updates when visibility is changed.

How Has This Been Tested?

👁

Types of changes

• Bug fix (non-breaking change which fixes an issue)

• Tweak (non-breaking change to improve existing functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.