DiffEditor: Refactor the user-facing parts

* Move all data handling into DiffEditorDocument
* Move much of the logic of how to update views into the
  DiffEditor.
* Introduce a base class for the different views on the diff
  to implement.
* Remove DiffEditorGuiController
* Make DiffEditorController smaller and merge the DiffEditorReloader
  into the class
* Simplify communication between the classes involved
* Make much of the implementation private to the plugin

Change-Id: I7ccb9df6061923bcb34cf3090d6d8331895e83c7
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@theqtcompany.com>

src/plugins/diffeditor/diffeditorplugin.h

@@ -44,9 +44,6 @@ class DiffEditorPlugin : public ExtensionSystem::IPlugin
     Q_PLUGIN_METADATA(IID "org.qt-project.Qt.QtCreatorPlugin" FILE "DiffEditor.json")
 
 public:
-    DiffEditorPlugin();
-    ~DiffEditorPlugin();
-
     bool initialize(const QStringList &arguments, QString *errorMessage = 0);
     void extensionsInitialized();