Add a new editor for editing translation items in PO files

[trinhanhngoc]

Updating translations for long and complicated messages like this is one of worst times in my Odoo developer career.

Idea about the UI for a new messsage edtior:

• Two panels side by side
• Left panel: original message without escape characters. Support syntax highlight (XML,...).
• Right panel: editable messsage without escape characters. Support syntax highlight (XML,...). When saving, message will be escaped to correct format.

cc @haumenphai @ngochuy97hp @anhbtit