steam: Fix big picture setup

[Drakulix]

Best reviewed commit-by-commit:

41be4c7:

These changes are enough to get steam in big picture mode completely initialized without first logging in without big picture mode enabled. No more black screen (fixed by having bluez), no more indefinite "Waiting for network" (fixed by NetworkManager). All services are running inside the container and are not exposed from the host to prevent steam from configuring host devices or network.

6bdf3e7:

Does the minimum to use the new big picture mode by default.

b9343cd:

Steals some parts of Chimera's gamescope-session or similar projects by enabling more parts of the SteamDeck UI. Not everything makes sense for games-on-whales given we are streaming however. So the list of adapted features is:

• Usage of Mango to display performance stats and enable the SteamDeck UI for showing them
• Usage of ibus to allow the Steam On-Screen Keyboard to be enabled when games require text input
• Enabling Steams Variable-Shading Menu. Not sure yet if it is working at all, but definitely depends on hardware.
• Better scaling support - including dynamically changing the resolution while playing. This should allow gamescope to properly resize steam, if we re-start the stream with a different resolution.
• Use Steam's Internal Browser for URLs
• Use multiple XWayland instances to separate Steam from the running games

I am not sure how much these changes degrade the desktop experience if the STEAM_STARTUP_FLAGS are overridden to not start in bigpicture. Also half these features require integration with gamescope and will be disabled if RUN_GAMESCOPE isn't set. As such it might make sense to drop this commit and create separate Steam Big Picture and Steam Desktop images (with the latter maybe using sway by default)?