[UX] Fix mix between default prefixes dir and shared prefix dir

[arielj]

This PR makes the final changes so it closes #3274

This renames the variables and setting keys we use to make it more clear that one is the default prefixes folder (where prefixes are created inside) and the default (now shared) wine prefix dir (the one used with the "Use shared Wine prefix").

The new setting value is only applied for new installation or if a user resets Heroic, for existing users this should not change anything (it means that if they already have the base prefixes dir as .../default it will continue like that and prefixes will be created inside it, to not break things for them).

Similar thing happens with the default wine prefix config, now it's renamed to sharedWinePrefix so it matches better the Use shared Wine prefix option. For existing user we still show the previous value they have (they probably have .../default) and for new users we show .../shared instead.

This will solve this issue where users end up with .../Prefixes/default as a prefix and also .../Prefixes/default/some game prefix here as other prefixes inside that one.

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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]

Tested here and I can confirm that it changes to shared only on new installs. On current configs it keeps using the default one.