UI: Refactor Virtual Camera source selector dialog

[tytan652]

Description

Prepare the dialog to be extended with new feature like #5383 by moving some code elsewhere.

Most of the "dialog" code does not belong there and was moved.

• Struct & enum moved to their own header.
• The config code moved to the main window code.
• The video mix code is moved to output handler.

Motivation and Context

Avoid putting those change directly in #5383 and allow a separate review.

How Has This Been Tested?

I tested:

• If the config was saved
• Starting the VCam with a source/scene/internal
• Changing the config while the VCam is active.
• Re-starting the VCam

Types of changes

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

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

Checklist:

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