feat: display layer change in a toast overlay

[rszyma]

The kind of overlay notification on desktop like the one sent by some keyboard keys state (capslock, num…). It would be great if we could set their position, text color, background color, size, position, duration or fixed.

Originally posted by @Fred-Vatin in jtroo/kanata#29 (comment)

[rszyma]

After some considerations I've decided that this doesn't belong to kanata-tray, as this project aims to be nothing more than a simple tray-app.

Although this will not be a part of kanata-tray, I like this feature idea and I can see using it myself. It's likely that I'll work on this in the future, just as a separate program. If that happens I'll notify about it here for sure.