* add LAST_MODIFIED attribute when exporting
* complement test_exporter for LAST_MODIFIED attribute
* parse LAST_MODIFIED attribute when importing
* use bookmark date_added when no modified date is parsed, otherwise use parsed datetime.
* complement test_parser and test_importer for LAST_MODIFIED attribute
* cleanup tests a bit
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Co-authored-by: Sascha Ißbrücker <sascha.issbruecker@gmail.com>