frontend: Add thumbnail size options to source select

[Warchamp7]

Description

Adds new SegmentButton widget and associated container/buttongroup class and implements it in the Add Source window for thumbnail size selection.

obs64_y2i7q8QRBs.mp4

Motivation and Context

Feedback on the Add Source dialog has included smaller thumbnails, or a list view again to reduce space used and visual clutter. This adds options for both to the window itself.

How Has This Been Tested?

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Types of changes

• New feature (non-breaking change which adds functionality)
• Tweak (non-breaking change to improve existing functionality)

Checklist:

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