Gk browser index in array column

Adds a non-editable column with index number for arrays.

@jpsim @tgoyne @alazier
In this case the reason for it to be -1 is not that it wasn't found, but rather because in array display mode everything is shifted because of the index column. So I'm not sure it makes sense in this case, but I will make sure that we use it when it results from a comparison.