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

Steam should not have access to my pictures

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

Steam has too wide access to my devices!

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

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