change Z order to put selected VHIs in front of other VHIs
change Z order to put selected VHIs in front of other VHIs

Fixes unexpected behavior involving overlapping helices (i.e. similar
x,y coords), whereby attempting to select and/or click&drag the
frontmost VHI while the rear VHI is selected will just change the
selection and move the helix to the back instead of dragging, e.g.:

![a](https://cloud.githubusercontent.com/assets/45583/17466348/ec843cc8-
5cc2-11e6-8771-349c736c5751.gif)

scholer/cadnano2.5