Add Proton-GE stable Flatpak as runner

[deadmeu]

Originally raised in https://github.com/bottlesdevs/programs/issues/276 it would be nice if you could choose to use the prepackaged Flatpak (com.valvesoftware.Steam.CompatibilityTool.Proton-GE) as a runner so that it is automatically kept up-to-date (and is shared with the Steam Flatpak).

This Flatpak was previously unmaintained, but has been recently picked up again.

[matheuswillder]

I was thinking that it was already possible to use Proton-GE Flatpak as a runner, because on the Bottles page on Flathub>Add-ons, it's listed:

Proton-GE (community build) Latest Proton-GE version built on the Flatpak Freedesktop SDK

But even installed, it really is not listed as a runner.

I also think it would be nice to add runners this way. While it may break titles if it receive automatic updates, manually updating runners and others components is tedious.