GRStepView: refactor according to old GRPadBank hack

[antonhornquist]

Better handle x and y separately than in the same method.

// TODO: from GRPadBank, better to handle x and y separately
prIndexToX { |index|
	^index % numCols
}
prIndexToY { |index|
	^this.bottommostRow-(index div: numCols)
}