Reverse the order of the panes in the main splitview

I found the ordering of the panes to be non-intuitive (in fact for the first 10 minutes, I was there trying to drag and drop files into the search pane). Master/Detail relationships in MacOS UI generally have the master (here, the files list) on the left, and the detail (here, the property editing) on the right. The search box is an adjunct/helper for the property editing. I get that search is the "main feature", but it just doesn't make sense to me that it's on the left when you'll need to add files before it does anything meaningful. Cognitively, I normally process things from left to right. The app seems far more usable to me with the files panel on the left.  Since there's clearly inertia behind the current layout, it seemed like it would be a low-impedance approach to have this behavior be governed by a user default.

I also simplified the NSSplitViewDelegate implementation considerably.