Process comments from xliff 1.2 files

Add the possibility to import comments (<note>) from xliff 1.2 files and export translate5 comments back to xliff 1.2

Disable/enable behaviour:

• enable: import and export is enabled
• disable: import and export is disabled

<note> tags can be on trans-unit, bin-unit and group level.

We import and export only on trans-unit level.