use gettext contexts for specific translations

Here is a little patch to add a parameter to the ts() function to specify a context (it is the term used by gettext).

Inspired from: https://developer.mozilla.org/en/gettext#Using_context_with_msgctxt

Let's take an example of french translation, with the string "Tag":

• the action "Tag" becomes "Tagger"
• the noun "Tag" becomes "Tag"

So I need to specify the context like that:

• ts('Tag', array('context' => 'action'));
• ts('Tag', array('context' => 'noun'));

In the .po file, I have:

msgid "action|Tag"
msgsrc "Tagger"

and

msgid "noun|Tag"
msgsrc "Tag"

Of course I can omit the context, and use normal msgid/msgsrc strings.

TODO: smarty

{ts}

wrapper, but perhaps it is automatic because I used the $params variable of the ts() function.