Rename DiffEditorWidget -> SideBySideDiffEditorWidget

It's the preparation for the new UnifiedDiffEditorWidget

Change-Id: I018899017c7e711ea8b04d85f8298b0d73f1cbab
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>
Fix a warning about no setFontSettings() slot

Caused by change 3fa6b6737c48049cb3d6e06ef0c1cfcb30dfd595

Change-Id: I4b1244423dfea83b1a23f6ecb03026956f14f35d
Reviewed-by: Eike Ziller <eike.ziller@digia.com>