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 (can't use music directly from Steam)

flatpak --user override --nofilesystem=xdg-music com.valvesoftware.Steam

Steam should not have access to my pictures (can't add images from pictures as profile pictures)

flatpak --user override --nofilesystem=xdg-pictures com.valvesoftware.Steam

Steam has too wide access to my devices! (may prevent controllers from working)

flatpak --user override --nodevice=all com.valvesoftware.Steam

flatpak --user override --device=dri com.valvesoftware.Steam 

Steam has devel permissions (may prevent some games from starting)

flatpak --user override --disallow=devel com.valvesoftware.Steam