Implement context to strings to translate

[Daniel-KM]

Translations in Omeka have no context, so in a record, the property dublin core : medium is translated in French "Support", but "medium" is used for thumbnail size too, where it is translated "support" too, but it has no meaning in this case. There are some other similar issues.

Generally, these issues are fixed by adding context to translation strings. There may be some other ways to fix it. It would be a good evolution for future versions.