UI: Added multiview selection border size settings

[Bemjo]

Description

This adds a new setting option the multiview group under the general settings, Selection Border Size. This option controls the size of the Preview and Program selection borders. It has a default of 4 which is the current unmodified setting in OBS. It has a maximum of 32.

Here are the default settings of 4:

Here are settings at 16:

Here are settings at the maximum of 32:

Here is a preview of 16 in a 25 scene layout:

Motivation and Context

It can be difficult to discern which scenes are currently selected in multiview, especially on more complex setups utilizing many scenes, this aims to improve visibility of the currently selected borders by offering the user a setting to increase the size of these borders.

How Has This Been Tested?

Yes, this has been tested to verify that border sizes properly change to those set 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.

[Warchamp7]

I'd rather improve the legibility of the borders instead of add yet another customization option.

A change for that was done in #10716