Allow setting icons instead of numbers

[martin-braun]

Similarly to #17 I'd like to personalize the desktops. I agree that custom names are not very suitable for the menu bar (too long), but what about icons?

I thought about something like CurrentKey Stats offers with monochrome icons or even emojis. Unfortunately, that program is not working well with multiple desktops, so I think this feature would be handy here.

Here a screenshot of the CurrentKey Stats:

The app is a mess and unstable, so I prefer WhichSpace, but numbers are not so useful.

[TheophileMot]

While we're at it, how about being able to assign custom background colours for the icons?