[Feature Request] Hide tray icon when no updates are available

Created by: meshy

Hi! First off -- thank you for all the work that's been put into this tool!

pamac-tray is the only tray icon active on my machine most of the time, and it inspires gnome shell to show this little box:

It's really useful when there are updates to apply, but as I don't use the tray icon to open the package manager, the icon is just a distraction for me the rest of the time.

I have added an option here to hide it when there are no updates, and would appreciate it if you would consider merging it. I have also tried to make it backwards compatible with the existing behaviour and defaults.

I've never written anything in vala before, so if there's any way that I could improve this for merging, I would appreciate the heads up. I'm very open to recommendations for better config/variable names too.

Hope this is handy!