Content selectors in 4.0

[lenhard]

In the current dev version, the content selectors are broken. This issue is just to remind ourselves that they have to be functional before we release 4.0. @tobiasdiez has this in mind, but judging from my own forgetfulness, I'd rather have an issue that tracks this ;-)

[koppor]

Documentation of that feature at http://help.jabref.org/en/ContentSelector

[lenhard]

I'll reopen this issue, but just to indicate that we have to adjust the documentation for content selectors that @koppor has linked above.

[lenhard]

And since it's so dead simple, I've done that just now. The updated documentation should be online in a few minutes.