Created ImagePanel, GNImages for image utilities.

Added File Chooser (uses system look and feel for the dialog)
- with image file chooser, already create some filters and preview

Minor changes in GNTable (and table model).
When editing, GNTable will check for getDefaultEditingString(). If null,
will use the editor's default (probably the object.toString()).

GNTableRowComparator will understand if rows to compare (the objects)
implement the Comparator interface, then use the compareTo.
Small fixes in JodaTime, Configuration, GNStrings and GNTable.
Create ErrorFieldSetter class.
Updated FormulaTextField to use ErrorFieldSetter instead of hardcoded
color change.