Add ::onDidDestroy to Pane and PaneAxis
:lipstick: pane-container-spec
Dispose of PaneAxis subscriptions when destroyed
Add PaneContainer::onDidChangeActivePaneItem
Upgrade event-kit to fix stupid bug
:lipstick: pane-container-spec
Actually subscribe
Remove/add the *subscription*, not the child
Add PaneContainer::onDidChange/observeRoot

Also, don’t use a behavior to monitor root changes