Allow addTimer to take a key value for uniqueness

[bitspittle]

Currently in Snake I'm doing something like:

var firstMove = true
level.snake.onMoved = {
   if (firstMove) {
      firstMove = false
      addTimer(KonsoleAnim.ONE_FRAME_60FPS, repeat = true) {
         ...
      }
   }
}

which could possibly be simplified to:

level.snake.onMoved = {
   addTimer(KonsoleAnim.ONE_FRAME_60FPS, repeat = true, key = level.snake) {
      ...
   }
}

In other words, if a timer with the particular key is already found, don't add it again.

[bitspittle]

It's way fewer lines, which is nice, so we should probably do it. But there's a workaround so it's not urgent.

[bitspittle]

Done. KonsoleAnim also benefted from the feature.