Default on-click behaviour

I was thinking that default on-click command should be exo-open --launch MailReader but I realised that some might prefer to update the plugin instead.

As a compromise, I think the best option is to have a drop-down with one option that updates the plugin and other that runs the exo-open command and opens the default Mail Reader application.

There are other plugins that open drop-down menu on click, so it is not very complicated to implement either.