Enhancement: toggle off with a click

I'm used to the toggle on/off behavior of other apps in the menu bar, and got a little confused when qmoji wasn't toggling off when clicking on the icon :) Could be a nice enhancement to have.

Start at qmoji’s menu-bar icon click handling and trace how the current on/off state is controlled. Update the behavior so clicking the icon toggles the application off as well as on; done means repeated clicks switch between the two states as shown in the attached example.

Written by the indexing model from the issue text.