frontend: Refactor and update Audio Mixer

[Warchamp7]

Description

Moves a bunch of logic out of OBSBasic into a central AudioMixer class, refactors a bunch of code to clean up, and adds a pile of new functionality to the mixer.

Adds a new MenuCheckbox widget for having a checkbox toggle inside a QMenu that does not close the menu after clicking.

The primary addition in this PR is a new audio monitoring toggle button in the mixer itself. Other features have also been added such as the ability to show Hidden sources, Inactive sources, adjust how they get sorted, and an easier way to toggle the layout.

PR also depends on #12734 for icon coloring.

Motivation and Context

With the fixes made as part of #12175, it is now far more reasonable for us to expose audio monitoring more directly to users. It is often very desirable to be able to toggle on monitoring for things like Media Sources or some Video Devices such as capture cards.

I elected to also clean up the Audio Mixer code a bunch while working on this feature addition, and add some other additional features and UI adjustments.

There is more that I want to do and clean up still but I'm trying to keep this PR at least moderately reasonable.

How Has This Been Tested?

Tested muting and monitoring on multiple audio sources both via the Mixer dock, the Advanced Audio Properties window, as well as a separate Audio Mixer window utilizing the same widget, to test that things are properly encapsulated and not kept in any local widget state.

Types of changes

• New feature (non-breaking change which adds functionality)
• Code cleanup (non-breaking change which makes code smaller or more readable)

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.

[gbdlin]

Small request for this redesign, if it's feasible and easy enough: can you add one more checkbox to the menu to show sources that are inactive, but present on the previewed screen? Of course it only makes sense in the studio mode, but it would be a great help when you need to adjust levels of a source that will become active with next transition, but without having to find it in the list of all available sources.

[Warchamp7]

Small request for this redesign, if it's feasible and easy enough: can you add one more checkbox to the menu to show sources that are inactive, but present on the previewed screen? Of course it only makes sense in the studio mode, but it would be a great help when you need to adjust levels of a source that will become active with next transition, but without having to find it in the list of all available sources.

This was in fact not straightforward but I added it as default behaviour