Allow GM characters in Group Manager

[obrenckle]

Is your feature request related to a problem? Please describe.
I'm playing in a GM-less game, both player have the GM status to have access to all of Foundry's tools, but that miss we can't use the Star Wars Group Manager for our characters.

Describe the solution you'd like
Have an option in the Group Manager configuration panel to allow GM Characters to be treated as Player characters.
I have an example implementation ready that I'll push later.