cmd/trayscale: support displaying connection status in menu

[slagiewka]

This adds a simple tray menu section that changes according to connection status and resembles the behaviour of the original client tray (macOS).

I found the current tray menu pretty lacklustre, at least compared to the official client (however, this is old design). In order to familiarise myself with the project, I decided to go with this simple info first.

Let me know if this is the direction you'd like to see to project go towards.