Add convenience methods to TextPane to deal with text-only documents:
* "getText()" that returns a String of all the text in the document
* "setText()" that constructs a document from the String consisting of
  a number of paragraphs, one for each line of the string.

These two methods allow TextPane to be used as a drop-in replacement
for TextArea, but where additional styling may be needed at some point.


git-svn-id: https://svn.apache.org/repos/asf/pivot/trunk@1519859 13f79535-47bb-0310-9956-ffa450edef68