We run by default with a panel (default gnome-panel) and then run Openbox
without any panel if one cannot be found.
- Adds a fallback session for if a panel (default gnome-panel) not found.
- Removes notifications as a requirement. Seems to be a legacy thing judging
from the ubuntu .session files. Notifications are being provided by
notify-osd on modern systems, and you can't check for its presence in
gnome-session (gnome-classic.session always fails because it looks for it, so
ubuntu falls back to gnome-fallback.session).
