Draft: Add a simple portal interface to communicate with Steam

For example, Steam could run this as a subprocess if desired.

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This is just a prototype, and hasn't been fully thought through.

Steam would ideally run this as a subprocess via steam-portal --exit-with-parent (optionally with --replace), so that it is automatically terminated when Steam exits.

The OpenKeyboard method would be enough to implement a GTK/Qt input method something like this:

• detect whether running under Steam (via an environment variable or something)
• if not, disable the IM
• if yes, when focus changes to a text input, call OpenKeyboard()

As prototyped here, it requires the steam-runtime-tools library, but it could be adapted to not.