How to add custom keybindings with gsettings
Issue solved in https://github.com/mate-desktop/mate-desktop/issues/561 :
$ gsettings set org.mate.control-center.keybinding:/org/mate/desktop/keybindings/custom0/ action 'pluma'
$ gsettings set org.mate.control-center.keybinding:/org/mate/desktop/keybindings/custom0/ name 'Pluma'
$ gsettings set org.mate.control-center.keybinding:/org/mate/desktop/keybindings/custom0/ binding '<Primary><Alt>p'