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.pro

@@ -5,8 +5,8 @@ HEADERS += diffeditor_global.h \
         diffeditor.h \
         diffeditorconstants.h \
         diffeditorcontroller.h \
+        diffeditordocument.h \
         diffeditorfactory.h \
-        diffeditorfile.h \
         diffeditorplugin.h \
         diffeditorwidget.h \
         differ.h \
@@ -15,8 +15,8 @@ HEADERS += diffeditor_global.h \
 
 SOURCES += diffeditor.cpp \
         diffeditorcontroller.cpp \
+        diffeditordocument.cpp \
         diffeditorfactory.cpp \
-        diffeditorfile.cpp \
         diffeditorplugin.cpp \
         diffeditorwidget.cpp \
         differ.cpp \