When adding a new timeline, the dialog should contain a description field

For now, when a new timeline is created, the associated dialog has a single field for entering the name. It should also contain a field for the timeline description.

In order to fix this, the dialog code in method Timeline.edit_properties should be included into TimeLineDialog. By the way, this class is defined in a separate file, dialog.py but should be moved into timelines.py. After that, the dialog.py file will only contain the classes ConfirmMessageBox and DialogCode. The former is only used once in tiempane.py, and could be moved into this file. The later is only used in files.py, but its usage could be replaced by QMessageBox.DialogCode.