Choose the Steam library to host the proton prefixes.

Feature Request

I confirm:

• that I haven't found another request for this feature.
• that I have checked whether there are updates for my system available that contain this feature already.

Description

Hi, I'm a still doing dual-booting because there are games that still doesn't work with proton/wine.What I'm suggesting here it's to add an option to Steam Play so we could choose where to have the prefixes generated (maybe choosing a steam library).

For example, I have a shared steam library with windows in NTFS formatted drive.
Some games worked when I tried to run them with proton in a NTFS partition, but links to simulated windows drives are created with ':' character so if you try to move the game full game installation, prefix included, this fails, and windows doesn't like filenames with the ':' .

As an example this is what I have done for Overwatch (the game) XD
The wine prefix is in ~/wineprx/ow/
with the path to the launcher ~/wineprx/ow/drive_c/Program Files (x86)/Battle.net
but for the battle.net launcher the game is in Z:\mnt\1TB\Blizzard\Overwatch (/mnt/1TB it's a NTFS drive/partition)

Justification

This will help people with dual-boot machines that want to share the same library between Linux and Windows, so you can have the prefix in ext4 formatted drives, and the NTFS drive that it's shared with windows has no illegal NTFS characters in files.

Maybe this has to be more with a Steam Play feature than a pure proton related thing because with external games I think, we can do what I'm suggesting here.

No files, tests, or entry points are named. Start by clarifying whether this belongs in Proton or Steam Play and how a selected Steam library should host prefixes while remaining compatible with shared NTFS game installations; done would mean users can choose a separate prefix location.