Improvement in deleting tabs

Until now if you had only one tab, the tab couldn't be deleted, since Ninja needs to have at least one tab. This has been uncomfortable when you wanted to delete the sample bookmarks. You either had to delete the category groups one by one or had to create a blank tab, go back to the other tab and then you could delete it.

From now when you want to delete the last tab, the content of the tab will be deleted and you will get a blank tab. This will make the removal the sample bookmarks pretty easy.