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 ?

headers/private/shared/AboutWindow.h

@@ -37,6 +37,9 @@ class BAboutWindow : public BWindow {
 			void			AddVersionHistory(const char** history);
 			void			AddExtraInfo(const char* extraInfo);
 
+			void			AddText(const char* header,
+								const char** contents = NULL);
+
 			BBitmap*		Icon();
 			void			SetIcon(BBitmap* icon);