1.5.6 released

Use jEditor 1.2.9
Use MDIUtilities 1.2.25.4
Use JSoup 1.14.2
Use epublib2 1.3.1
Fix some cases where the pre syntax thrown an exception
Allow to use external files for the pre content
Fix references not taken into account in messageBox elements
Add a menu which allows to generate a configuration properties file from the GUI
It is now possible to use the "input" text field in the GUI to specify the input directory
Fix the handling of the chapter element
Allow to create images with a transparent background
Support the cover option in the configuration
Add the "progress" type for messageBox