Add a line number / character count ruler for use with scrolling text areas

The idea is to take the Ruler class that is already implemented in two of the tutorials and beef it up so that it can act as either a line number ruler (vertical orientation) or a character count ruler (horizontal orientation) for use with TextArea or TextPane inside a ScrollPane. It would be attached either as the "rowHeader" or "columnHeader" of a ScrollPane with the appropriate orientation.
By selecting the same font into the ruler as is used in the TextArea or TextPane, presumably it would be in sync with the text. Alternatively, we could add row height and column width styles that would override the font metrics if there was a sizing problem. Will have to see once I get it implemented.
There could be other possible useful styles – again, I will have to see once I get it implemented what might make sense.