Add remote-event functionality. Fixes #51
This merge request adds remote-event functionality to the mailwatch plugin, as exists in some other plugins. With this, you can programmatically force a refresh of the plugin via:
xfce4-panel --plugin-event=mailwatch-X:refresh:bool:true
...where X is the widget ID number.
You can also integrate this in the "Run on Click" option to refresh the plugin automatically, and/or after running and closing your email client:
sh -c "gmail-desktop && xfce4-panel --plugin-event=mailwatch-2:refresh:bool:true"