[FIX]ir_translation: replace source field to show value of the term stored in db and reflect change directly
in db when changing source value
[REF]ir_translation: improve _get_src and _set_src function
[IMP] ir_translation: GUI improve in order to be able to change the english value of a model field (eg: a product name) directly in the translation window + small refactoring in order to test 'if context is None' rather than 'if not context' (see lp bug 525808)
[FIX] ir_translation: fix in new methods made to improve the usability, see previous commit
[FIX] ir_translation: fixed erro 'context_wo_lang is not defined'