Add Hide/Show Unreal Marketplace settings + some fixes

[flavioislima]

• Add setting to show or hide Unreal Marketplace on the Library.
• Fix Alternative Legendary Path not being recognized #781
• Implement Enabling overlay scrollbars for a much cleaner look #691

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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)

[flavioislima]

Tests are not working because I could not find a way of proper mock electron-store on the frontend. Would need some help @Nocccer