The system tray application I want is simple, yet I've had no success finding one.
It should alternate between two (or, as a bonus but not a requirement, more) icons depending on input it received. For example, the icon could be determined by piping
2, ... to it. My intent is to use that to make a blue/black square if my laptop's touch-pad is on/off and, similarly, to monitor the state of modifier keys (e.g. CapsLock), and some other related scenarios I can think of.
A nice example of something close to what I want is
It lights on or off each square based on whether ScrLock/CapsLock/NumLock is activated or deactivated. However, it is a normal window, not a tray application, and it will only detect keyboard keys state.
If the application I described does not exist, I would still appreciate any suggestion that comes close to it.