Reduce duplication of content in KeyboardHelpDialog

[jessegreenberg]

In #847 @pixelzoom pointed out how how content in the KeyboardHelpDialog can be unnecessary and repetetive, noting

'... it would be more usable and scalable to expand the "Basic Actions"'.

@terracoda @arouinfar FYI, and Ill add this to the alt input project board.

[kathy-phet]

Related: #856

[arouinfar]

One suggestion is to include the Alt+K hotkey for PlayPauseButton in the "Basic Actions" section rather than factoring it out into "Timing Controls".

It's possible the "Timing Controls" section stems from Molecules and Light, which was the first accessible simulation with a TimeControlNode. MAL has a sim-specific hotkey to step forward in larger amounts. We have not repeated the Alt+L hotkey for any standard StepForwardButton.