Debugging activities feels weird when time is irrelevant

Example: https://code.world/#P78hkv9hsakKxPJw-eO5LYw

Playing around with this, it's quite confusing that:

1. You must be paused to access the history, even though time has no effect on the program.
2. You must be unpaused to press the space bar and advance the simulation.

It's not completely clear what to do here. One option is to use the same trick we do for optimization, and just show the same controls as if the program is paused, any time the program ignores `TimePassing` events. (One exception is `PanningLayer`, which must be disabled if the program responds to mouse clicks, but could be left enabled while paused for a program that changes over time.) We could use `isUniversallyConstant` in `CodeWorld.Driver` to check both properties.