Revert "Merge the react and non-react editor styles"

This reverts commit 01c4fe53403978f077d7b43bff8fb2cfec65fd17.
Use RxJS observables for an event and a behavior on Pane
Use getters in pane-spec instead of direct property access
Implement Pane::isActive without behavior
Convert pane-container-spec to not reference raw properties
Add PaneContainer::observeActivePane and Pane::observeActive
Add Pane::onDidRemoveItem()
Add Pane::onDidMoveItem
Add Pane::onDidActivate
Add null guard
Add Pane::onWillDestroyItem
Remove Pane::$active behavior

It's been supplanted by ::observeActive
Use getters/setters for parent and container of panes and axes
Use setActivePane when setting root of container