Use split window more effectively for browsing maps

When following topic-to-topic links in a split window, the focus changes to the target topic in the active half.

It would be very useful to target the other half of a split window, so that one half could be used for top-level navigation of a map while the other lets you examine what is at the other end of the link.

1. Right-click on a topic-to-topic link and click on a new command "Open in split window" (active for topic-to-topic links in the same map only)

2. The target topic scrolls into view in the other half of the split window, while the left half stays in the same place for more navigation.