Split View Polish

@redmunds I thought about it last night and I don't feel comfortable highlighting the entire pane header if the pane is active. I've added the selected styling to the selected items in the working set and made the active pane header text more obvious by making the inactive pane header quieter.
Using `#second-pane` works for now, but not sure what the naming will be when more than 2 panes are allowed. I tried to make a selector that applied to "all elements except first" with `::nth-child(n+1)` so it would scale to > 2 views, but I couldn't get it to work.