Add modern user agents for common browsers

Review Request #113608 - Created Nov. 4, 2013 and updated

Maarten De Meyer
I recently needed to swith user agents to (very) new browser versions. This is an attempt to add them upstream.
Since browser makers release new version every other week it seems useless to make a new file for each one. I just named it latest and don't include the version number in the description.
That way translators have no extra work when we update the user agent.

David Faure
We definitely need an update of the UA browser versions indeed, but I'm a bit unsure about the use of "latest" to replace version numbers.
As a user, it means I have no way to see if this is using some recent version number or some old one because we again forgot to update it.
So if e.g. the website says "your browser is unsupported", it becomes harder to know what the problem is.