Preview tabs shouldn't issue warning when closed after parent folder is deleted

[adamreisnz]

Consider the following scenario:

1. Single click a file in tree view to preview
2. Look at contents of file, decide you don't need it anymore
3. Right click the parent folder of that file in tree view and delete it
4. Single click on another file in tree view
5. Annoying alert pops up telling you the first file has changed and if you want to save the changes

I reckon that that warning should be suppressed for preview tabs. When you make changes in a preview tab, it becomes permanent, and then when you try to close it it makes sense to get a warning. However, for temporary preview tabs which you haven't changed, this warning doesn't make sense.