frontends: XMLUI refactoring + dialogs: - there are now XMLUIPanel and XMLUIDialog both inheriting from XMLUIBase - following dialogs are managed: - MessageDialog - NoteDialog - ConfirmDialog - FileDialog - XMLUI creation is now made using xmlui.create(...) instead of instanciating directly XMLUI - classes must be registed in frontends - "parent" attribute renamed to "_xmlui_parent" to avoid name conflicts with frontends toolkits