[email protected]: Add option to hide tray icon when there are no unread emails

[victor-marino]

Mentioning @hyOzd as he's listed as the spice author here, as well as @clefebvre since I can see he's responsible for much of this code.

This small PR adds the option to have the tray icon only show up when there are no unread messages, otherwise it removes itself from the panel.

I find this really handy to de-clutter the panel, as showing a persistent icon saying you have zero emails isn't very useful in my opinion. Also, having it appear only when there's a new email makes it stand out more, which I find works even better as a notification tool.

I've set its default value to false so that it doesn't confuse people when they update and the icon suddenly disappears.

I have also translated the new strings to Spanish since that's my first language, and took the chance to correct a small typo (léido -> leído).

P.S.: Apologies for the polluted diff... VS Code reformatted everything on save and changed the indents, new lines, etc., so now everything shows up as modified. Hope it's not an issue 🙏