Config: Convert Ozone 'Color Theme' setting to array

[Ryunam]

Description

This PR proposes a conversion of the Ozone "Color Theme" setting (which was previously based on integers) to an array-based setting, using string identifiers instead of numerical values (ie "basic_white" instead of "0").

I've been thinking about introducing this type of change for a long time, ever since discussing the viability of it with jdgleaver back in 2021. The main advantages are the following:

• Themes can now be re-ordered without disrupting existing configurations at all and new additions can be placed in-between existing themes in the list. This was the main driving force behind my intent: I am planning to add some new "Basic" color themes to Ozone that will complement "Basic Black" and "Basic White" and will be placed right after those two.
• With string identifiers it is arguably easier to change the desired value of the setting manually in the retroarch.cfg file, as opposed to having to remember which number is mapped to which theme.

Most importantly, this conversion is backwards-compatible and will not break existing configs. The code in configuration.c takes care of reading legacy numeric values assigned to Ozone themes (if present in the user's config file), setting the desired theme as expected, then converting that legacy number in the config file to the new string-based value.

Reviewers

@LibretroAdmin