[UX] Allow setting the Disable UMU default in General Settings > Game Defaults

[arielj]

This PR adds the Disable UMU setting in the defaults in general settings so it can be configured for all new games added instead of disabling it manually for each game if the user doesn't want to use umu at all.

This doesn't globally disable umu though, for games that are already installed it has to be manually disabled.

I also added some tests for the global config and game config to start adding tests for these things when we change defaults and that (it's not covering all settings and many cases, but it's a start)

Since it doesn't exactly disable UMU globally, it doesn't fully solve #4374, but I think it's good enough since the option to change the default value aligns more with how the other settings work.

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Use the following Checklist if you have changed something on the Backend or Frontend:

• Tested the feature and it's working on a current and clean install.
• Tested the main App features and they are still working on a current and clean install. (Login, Install, Play, Uninstall, Move games, etc.)
• Created / Updated Tests (If necessary)
• Created / Updated documentation (If necessary)