Sandbox hardening

By default this application does not have a very strict sandbox. There's been suggestions on how to isolate it further but these items have not been included by default due to compatibility concerns

Steam should not have access to my music

flatpak override --no-filesystem=xdg-music com.valvesoftware.Steam

Steam should not have access to my pictures

flatpak override --no-filesystem=xdg-pictures com.valvesoftware.Steam

Steam has too wide access to my devices!!!

flatpak override --no-device=all com.valvesoftware.Steam flatpak override --device=dri com.valvesoftware.Steam