split pane changes

[michaelhelper]

No description provided.

[curran]

Closing in favor of #831

Some good ideas here! Specifically

for (const child of activePane.children) {

We'll need this sort of logic in the new SplitPaneView component! More like activePane.children.map(child => <SplitPaneView/>) or something - a recursive React component, which is super cool.

Also the

 // TODO Copy the tab list from the active pane

is totally valid and still needs to be done. Cheers!