Making order plugin working when list has joins

The limitation that enabled list order plugin only with one single "order by" element prevented reordering also in case when the list had joins (even with a single order by element).
At the same time the lists with joins often have to be ordered by multiple elements as we may want to display the joined data always in particular order (let's say composer always the first, lyricist the second etc).
Corresponding changes in help text
Corresponding changes in et-EE help text
Merge pull request #1302 from jaanusnurmoja/patch-45

Making order plugin working when list has joins