Fix Copy Template and Rename File buttons if Hathor is uninstalled

Based on a JSE question: http://joomla.stackexchange.com/questions/2501/where-is-the-copy-template-button-in-template-manager-in-j3-3-1-after-update

### Issue
If you uninstall Hathor, the "Copy Template" and "Rename File" buttons in the template manager is no longer showing up. This is because the current check explicitely checks for `0`. The database query however returns `null` if Hathor is uninstalled.
The check is needed because Hathor has special overrides where those features are offered in a accordion instead of a button. Thus the button should be hidden if Hathor is the active template.

### Solution
Since we can get the active template name using the API `JFactory::getApplication()->getTemplate()` I have changed the code to use this, only showing the button if it's not "hathor".
I also removed the model method `getHathor` and the view property `hathor` because they are no longer needed. With this approach it should even use one less database query.

#### Testing
* Check in Hathor that the buttons still don't show.
* Switch to Isis and uninstall Hathor and check the presence of the mentioned buttons in the template manager (when you edit a template or a file).