Fix bug that the GUI does not update anymore

When a window is closed the application view was not updated
anymore. Two small bugs lead to this effect. Fixed now.
Remove item.files

The files attribute based on Files was much too complex for a
simple tasks such as delete an item. The file_ops plugin still
needs some work, but it does not need the complex Files(), it
can gather everything from the item itself.

Note: files.py was already deleted in the prev. commit.
Remove redundant mode attribute