Extract FileDiffViewer in the widgets module

Author: MightyCreak

.pylintrc

@@ -100,6 +100,7 @@ disable=raw-checker-failed,
         too-many-arguments,
         too-many-branches,
         too-many-instance-attributes,
+        too-many-lines,
         too-many-locals,
         too-many-nested-blocks,
         too-many-statements,