callback function for file browser

[AlexW]

Allow callbacks for the file browser functions.

e.g.

CKEDITOR.config.fileBrowserCallback = function(selected) {
    //Load file manager etc.
    //File selected so let CKEditor know.
    selected('my selected file');
});

[Jakub Ś]

DUP of #6085

@AlexW it is enough to leave idea in one ticket. You shouldn't have created new ticket for version 4. We will have this fix either for v4 only or for both.

Please also note the version field indicates where problem started occurring and not where it will be fixed (milestone is for that).