UI: Composable Themes System

[derrod]

Description

This PR introduces a system that allows "layering" of theme files, i.e. a base theme can provide styling, a variant theme the colour palette, and optional high-contrast adjustments or user customisation can adjust it to suit the user's needs.
Themes will be able to specify variables that can be overwritten and extended by child themes, to provide much more flexibility than the Qt palette system currently in use, as it allows arbitrary values to be specified.

Bonus feature: Loaded theme can be watched for changes and automatically reloaded when edited (hidden config option [Appearance] -> AutoReload).

Also see: Overview of syntax and design goals (will be migrated to wiki/design repo once finalised).

Additionally, theme settings are moved to a new "Appearance" tab in the settings menu, which will be expanded upon in the future:

Motivation and Context

This PR is based around conversations with @Warchamp7 on how we can improve theming in OBS and better accomodate new UI elements in the future.

An additional benefit for third-party themes is that they can inherit stock OBS themes and adjust them to their needs without having to copy everything into their own file. This also should make them forward-compatible as styling for the UI elements introduced in PRs such as #8446 will automatically be inherited and adjusted to the themes colour palette.

How Has This Been Tested?

Built a basic test base theme (Yami.obt) and variant theme (Yami_test.ovt) to validate parsers, UI, and composing to be functional.

Types of changes

• New feature (non-breaking change which adds 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.

[derrod]

Based on feedback by @Warchamp7 I removed the UI for customising variables for now and cut the PR down a bit. We will revisit that later on once we're comfortable with the composition system and its capabilities.

Edit: Themes are still unfinished, I'll leave that for Warchamp to solve :)

[RytoEX]

A quick skim. Will set aside some time for building/testing later this week.