Add an option to change the global Proton version

[Tiagoquix]

Is your feature request related to a problem? Please describe.
Hi. I would like to have an option to change the global Proton version, in a similar way to what it is currently possible to change the Proton version used by specific games.

Describe the solution you'd like
Add an option in the main screen to "Set this version as global" or something like that.

Describe alternatives you've considered
I don't see any other alternatives. Maybe also add a warning for the user?

Additional context
I don't know how this would work if the two "Compatibility" options weren't enabled in Steam's settings. I assume it would be the same as for specific games, which (I believe) enable the option to force such Proton for such specific game if it is disabled.

Also, see ValveSoftware/steam-for-linux#10877.

Thanks in advance.

[sonic2kk]

Already being worked on for Steam with #336. Albeit not on the Main Window as I don't know if this makes too much sense from a UX perspective. It's on the compat tool info dialog.

For Lutris, I did some tinkering around in the past to find out what would be needed and I'm hoping to do a follow-up PR after that one.