TreeView: Don’t get CSS row separator height >once

We were looping the columns and, if is_separator, assigning from get_separator_height() on each iteration. We should just do it once, so we don’t go through a lot of CSS machinery for each column for no reason

I've not profiled this or anything, but it seems like a fair bit of work going through CSS stuff over and over again, which we can avoid, given that the value can't differ between columns.

(also applicable to gtk-3-24 if accepted)