Add Pane::onWillDestroyItem()
Add PaneContainer::onDidChangeActivePane
Add Pane::onDidChangeActive()
Subscribe with new event methods in PaneView
Add Pane::observeActive and ::observeActiveItem

These have behavior semantics, invoking the observer immediately with
the current value of the observed property.
Don’t invoke ::onDidChangeActivePane events unless it really changes
Don’t emit ::onDidChangeActiveItem events unless it really changes
Use Pane::setActiveItem internally so observers are invoked
:lipstick:
Handle objects in subscriptions instead of multiple args
Use getActiveItem getter in pane-view-spec