Engine's implementation of Simulation.goToStep not functioning correctly when called in FXUI

[Vuksaa]

The implementation of goToStep could be bugged. The way it is currently called by the FXUI produces some odd behaviours.
To observe the issue:

1. Start a simulation with the -fxui flag.
2. Play the simulation briefly, then pause it.
3. Press the key corresponding to the "one step forward" action - this calls goToStep and passes it the current step + 1.
4. The simulation does not move forward one step; instead, now the play/pause feature will move one step ahead, and occasionally crash the simulation.

I remember this specific sequence of actions was working as intended back when I implemented it in the FXUI, I'm not sure if it's an issue in how the FXUI calls goToStep or if the functionality of goToStep was changed. If it's the former, I can attempt to fix it through a PR, though I already tried tweaking it and couldn't get it right.

On a side note, the implementation in Engine.goToStep is different than what is documented in the javadoc for Simulation.goToStep:

• in Engine, it attempts to pause the simulation when the current step is higher than the given argument.
• in Simulation, it is described as if it executes a number of steps given as argument instead.