UI: Added accessibility colors for multiview selections

[Bemjo]

Description

This adds two new accessibility color settings to allow user modification of the two selection border colors in multiview, the Preview color and the Program color borders.
The colors have been placed under the current Selection Border settings, and above the Mixer Volume colors, to group them together with similar settings.

Motivation and Context

I believe having accessibility settings for these colors can help improve visibility not only for the visually impaired, but for those who may find the default colors difficult to see against their current scene setup. This allows for flexibility for the user to choose what is comfortable for them.

How Has This Been Tested?

Yes, this has been tested to verify that colors properly change to those set in the accessibility settings under the following scenarios:

1. When loading OBS and opening multiview.
2. Changing from the settings, and then opening multiview.
3. Changing from the settings with multiview actively open.

This has only been tested on Windows 10.

Types of changes

• New feature (non-breaking change which adds functionality)
• 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.