Load notes from xliff files

Parse XLIFF files to extract "note" property and store them into the messageCatalogue metadata
Dump notes into xliff file

Retrieves notes from messageCatalogue metadata, and store them into xliff file
Copy metadata in DiffOpertation and MergeOperation

Some information contained in source catalogue (like Xliff's notes) have to be write in target catalogue.
Use comparison against null instead of is_null