Implement the ability to ignore certain controllers for Steam/Proton

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

I have been using Antimicro to do some custom controller mapping and scripting in certain games, and what bothers me is that Steam can not be told to ignore a certain controller I have plugged in. I used to get around this by unplugging a controller and replugging it, so that Steam no longer picked it up and my Antimicro profile worked correctly, but as of recently, this is no longer possible.

Justification [optional]

Proton is the thing I triggered this bug with, and the one which later fixed it.

Risks [optional]

References [optional]

Start by investigating how Proton handles controllers and how Steam identifies the controllers it exposes. Determine where an ignore list could be configured and clarify the expected behavior for ignored versus available controllers. Done means selected controllers are ignored by Steam/Proton while the requested controller mapping continues to work.