fix: Don't attempt to hide the window if no active status icon present

[City-busz]

If no status icon is visible, the window cannot be restored, leaving a process running in the background. Instead of that, just exit the app on close in this case.

[C0rn3j]

I've done something similar in 727d3dd

First, am not sure about removing IS_UNITY - I thought Unity is dead but apparently it's revived again in Ubuntu Unity - with the added check I suppose it might be safe to remove the IS_UNITY check?

Second, I think the original PR is bad UX - user already configured what's effectively "Do not close when I press X" and the PR ends up closing on X if tray is broken - I've changed it to do nothing and log an error, which I think is better and also immediately shows that a bug has occurred, rather than having the app effectively crash without the user knowing and expecting it to be there in the background.

[City-busz]

I think it would better to show a confirmation dialog like this: "Unable to hide the window to the system tray. Do you want to quit the application instead?"

[City-busz]

See: #114