Fix spurious prefix/suffix helpers

I see, that you have been struggling with this before :)
I can still get a couple of additional down-arrow suffixes for list fields on multi-tab forms.
(See http://sw-amt.ws/w2ui/demos/#!grid/grid-31 or http://jsfiddle.net/rYL5F/13/).

This is the last fix I need for my first real-life production application
employing w2ui. Everything else works out of the box now. (If you are
interested, you can see it at http://sw-amt.ws/trade_cat/ for a while).

Thanks for your great effort!
Merge pull request #476 from wolfmanx/fix-form-list-field

Fix spurious prefix/suffix helpers