Add AddText method to AboutWindow

 * Factor out the code to add some data to the about window, with a header and a content under it
 * Make this method public so it's possible to add custom entries in an about box
 * If the method is called with only the header or only the content, the text is added non-bold and non-indented (like the description entry*).
 * Make the header text bold. I'm not sure it looks that good, after all. Thoughts ?