Add ZED_ALWAYS_ACTIVE env var, use it in local collaboration script

This makes zed always behave as if the app is active, even if no window is focused.
It prevents the 'viewing a window outside of zed' state during collaboration.

script/start-local-collaboration

@@ -44,6 +44,7 @@ position_2=${half_width},${y}
 
 # Authenticate using the collab server's admin secret.
 export ZED_STATELESS=1
+export ZED_ALWAYS_ACTIVE=1
 export ZED_ADMIN_API_TOKEN=secret
 export ZED_SERVER_URL=http://localhost:8080
 export ZED_WINDOW_SIZE=${half_width},${height}