Watch for active pane item change in workspace center only

If we watch for changes on the workspace then we don't get an event in the case that an already-open file is clicked in tree-view, because tree-view remains the active item

lib/watch-workspace-item.js

@@ -113,8 +113,7 @@ class ActiveItemWatcher {
   subscribeToWorkspace() {
     this.subs.dispose();
     this.subs = new CompositeDisposable(
-      this.workspace.onDidChangeActivePaneItem(this.updateActiveState),
-      this.workspace.onDidDestroyPaneItem(this.updateActiveState),
+      this.workspace.getCenter().onDidChangeActivePaneItem(this.updateActiveState),
     );
     return this;
   }