simplify enum constants; remove unused code

[Jeepston]

Purpose

Simplifies usage of enum constants; removes unused code

Approach

• convert all read-only enum constants from cases to static let variables, which allows to drop usage of .rawValue in all places where that constant is needed
• removed unused constant enum related to artwork theme colours, since everything theme-related was moved to SimpleTheme

[GianniCarlo]

@Jeepston It looks like the build is failing because the PlayerSettingsViewModelTests.swift and PlayerManagerTests.swift still use the .rawValue

[Jeepston]

Ah yes, I knew I've missed something :D For some reason, project didn't have Test scheme generated. After adding and compiling for tests, I was able to fix it.