In the process of creating a more generic document interface manager (or window manag...

* Now adds panels instead of windows - one becomes part of the main windows; others act as separate windows.
* Can manage multiple windows
  - when the main window is closed, closes all the other managed windows;
  - when the main window is minimized/restores, does the same for all the other managed windows.
* XML file attribute 'mainPanelName' is used to set the main panel; if no attribute is set, uses the first panel.
* Panels can be deleted externally; a garbage collector makes sure that the list of managed windows does not grow
  forever if somebody decides to add/delete millions of panels.