Access to session mode details

[Ron Yorston <rmy frippery org>]

In my Frippery Move Clock extension I access SessionMode._modes to
determine if the clock is supposed to be in the centre panel in the
current mode.  In GNOME Shell 3.26 this results in a warning that
this access is not standards-compliant and should be fixed.

I haven't found a satisfactory way to get the information I want
without accessing SessionMode._modes.  Any ideas?

Ron