Description of the new feature/enhancement

Currently when you using split-pane commend with wt.exe, each successive split by default further splits the new pane. This is it is not possible to split my screen into 4 quadrats of equal size.

Proposed technical implementation details (optional)

It would be nice if split-pane command would accept an optional target parameter (similar to focus-tag command) specifying which page we want to split.