Take care of supernumerous $destroy events

See issue #30

Should avoid errors like: 
```
TypeError: Cannot read property 'removeChild' of null
    at TreeViewItem.destroy (chrome-extension://aadgmnobpdmgmigaicncghmmoeflnamj/ng-inspector.js:494:27)
    at chrome-extension://aadgmnobpdmgmigaicncghmmoeflnamj/ng-inspector.js:1190:11
    at Scope.$broadcast (http://localhost/OutlinerModel/lib/angular/angular.js:13082:28)
    at Scope.$destroy (http://localhost/OutlinerModel/lib/angular/angular.js:12734:14)
   ...
```