Polyseat 0.35.0

A new seat could not be signed in to Steam. Steam runs inside gamescope,
and gamescope only shows the windows Steam tells it to, which Steam does once
Big Picture is up. A Steam nobody has signed in to has no Big Picture, so its
sign in window existed and was never on the screen: picking Steam in Moonlight
gave a black screen, in every seat built since 0.30.0. Seats signed in before
that never met it. Picking Steam now puts that window on the screen. After
signing in, pick Steam again if Big Picture does not follow by itself.

Steam in a seat's launcher does something. Started from the grid on the
desktop, Steam opened on the other workspace, where it lives, and the desktop
stayed as it was. The entry now switches to Steam and opens Big Picture, the
way picking Steam in Moonlight does.

Seats have to be built again for this: recipe generation 62. Both are
files inside the seat, and a seat built before keeps the old ones until it is
provisioned again, which restarts it once.