Handle naming collissions during rename more gracefully

[petterh]

The UI gets mighty confused if we try to rename a file to something that exists, either as a file or as a folder. We should handle that gracefully (see Document::setPath).

This work item was migrated from CodePlex

CodePlex work item ID: '33968'
Vote count: '1'

[petterh]

[petterh@10/27/2016]
If you rename Foo.txt to Foo, and the folder Foo exists, the file is moved into Foo, still named Foo.txt. The UI, however, thinks the file is named Foo, and displays a folder icon for it.