Update Legendary. Replace XDG_CONFIG_HOME variable. Disallow uninstalling epic game if installing one

[arielj]

This PR is a replacement of #2806 and #3137

It includes the update of Legendary and the change to use the LEGENDARY_CONFIG_PATH env variable and also adds the dialog that disallows users from uninstalling an Epic game while an Epic game is being installed since that's not possible with the latest Legendary.

cc @Etaash-mathamsetty and @CommandMC

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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]

I am fine with this solution for now.
Although we should think on improving it in the future maybe having more control over the epic games and using legendary mostly to download the games itself, but with Heroic handling the install.json file.
I know there is a way of only downloading the game with it so this way Heroic would handle both installing and uninstalling.

[flavioislima]

Tested here and everything seems to works fine.