[Linux] Search for Proton on all steam libraries

[flavioislima]

Added a helper function to read from libraryfolder.vdf and add the libraries to the steampaths

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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)

[CommandMC]

Looks good in general, left some comments but they're not crucial