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>

src/plugins/diffeditor/diffeditor.qbs

@@ -17,10 +17,10 @@ QtcPlugin {
         "diffeditorconstants.h",
         "diffeditorcontroller.cpp",
         "diffeditorcontroller.h",
+        "diffeditordocument.cpp",
+        "diffeditordocument.h",
         "diffeditorfactory.cpp",
         "diffeditorfactory.h",
-        "diffeditorfile.cpp",
-        "diffeditorfile.h",
         "diffeditorplugin.cpp",
         "diffeditorplugin.h",
         "diffeditorwidget.cpp",