Rename DiffEditorFile -> DiffEditorDocument

The old name had nothing to do with a file in fact.
The new name will be more appropriate for the new stuff,
which will come soon.

Change-Id: I17ad03dab5c643ff26e313a434ba86929a0f00c3
Reviewed-by: Eike Ziller <eike.ziller@digia.com>