Ensure that the parents of deleted tree nodes remain directories in the file tree

There is apparently a bug in jsTree in which deleting an entry can cause its parent to be rendered as a leaf instead of as a directory. A workaround already existed for this in Brackets, but it relied on an assumption that the deleted item would always be selected. This is no longer the case because items in the tree can be deleted as a result of filesystem events. This pull request fixes this by manually setting the class of the node that corresponds to the parent entry when it has no children.