Last modification date is not exported (1.5.4)

Reproduce:

1. Create new portable CN 1.5.4 installation

2. Create some notes, stepping some time intervals
3. Sort by modification date, reverse order
4. Modify the oldest note, so it pops up
5. Export all notes to .xml or .txt
Result: modification date is not saved in exported files, so importing them into a clean CN would lack modification date information.