Global hotkey for the StepForwardButton should be moved to common code

[jessegreenberg]

There is currently a hotkey added in molecules-and-light that steps forward the simulation when pressing alt + "l". This should be moved to common code. Most likely to StepForwardButton.

[pixelzoom]

Related to #682.

[pixelzoom]

If move to StepForwardButton, it should be the default hotkey, not the only possible hotkey. As we concluded in our Slack discussion about hotkey support in common-code:

Provide a nice default that fits most contexts. Allow the client to override to fit a different context.

[zepumph]

Subset of phetsims/scenery#1298

[jessegreenberg]

This would be done with #682 , we don't need to track this in its own issue. Closing.