1.2.0: Install every extra with the .deb; find gamemoderun in /usr/games

- .deb now depends on gamemode, network-manager, wireplumber, bluez,
  ddcutil and libglib2.0-bin (were recommends or missing), so GameMode,
  Wi-Fi, volume, controllers, monitor brightness and the taskbar pin work
  out of the box
- Load i2c-dev at boot (and right away outside a chroot) so ddcutil can
  reach external monitors; its udev rule gives the logged-in user access
- Debian/Ubuntu put gamemoderun in /usr/games, which isn't on PATH in
  every session: look there too and run it by full path
- Install test checks every extra arrives; policy-rc.d keeps the test from
  starting services on the machine running it

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MYkT7n8s1tHWQQTfEQPV6o