[Shell] Propose telling addins to use the other overload.

This creates a Gtk.Menu on Mac, which means we don't make use of the awesome native Menu we have implemented in the other overload.
Propose hiding this API (not fully removing it).

/cc @slluis @mhutch @alanmcgovern @bratsche @DavidKarlas 
Enable more context upwards to see what I'm talking about.