Restricted filesystem access

As it stood, Bforartists had unrestricted access to the host system.
This is unacceptable, so now Bforartists only has access to home and
/tmp.

/tmp is needed for autosaves. It was found in
flathub/org.blender.Blender#122 that using
Flatpak's remapped tmp directory causes issues with other Flatpaks not
starting.
> Blender autosave files shouldn't be saved to the user's /run folder
> as it's very restricted with regards to available space. Once it's
> filled other programs are likely to fail to start.

Blend files get massive, especially for multiple autosaves, so access to
/tmp (which is cleared on reboot) is important

de.bforartists.Bforartists.json

@@ -11,7 +11,8 @@
         "--socket=pulseaudio",
         "--device=dri",
         "--share=network",
-        "--filesystem=host",
+        "--filesystem=home",
+        "--filesystem=/tmp",
         "--filesystem=/run/spnav.sock:ro",
         "--env=SPNAV_SOCKET=/run/spnav.sock",
         "--env=TMP_DIR=/tmp",