[FIX] invoice_xunnel: Non emitter partner in SAT XMLs.

Wizards:

• The file wizards/attach_xmls_wizard.py was included into the init file.

Models:

• IrAttachment.validate_and_update_description method created to hold already existing logic and expose it.
• Now XMLs from version 4.0 are also treated as xunnel ones.
• When creating documents, the attachment description is created explicitly.

General:

• Module version increased.