Include UUID for each item in export

[gnusupport]

Imagine me who used multiple devices to take various notes, and desktop software to accept and import such notes.

A note must be very unique and not just by date of creation.

Other programs must be able to recognize if note already imported should be now updated with the new version, or disregarded, or interested as new.

To solve those issues with multiple devices and programs using the exported file, please add unique UUID to each item.

That way other programs that import notes will recognize and sort it correctly.