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/diffeditor.qbs

@@ -22,14 +22,10 @@ QtcPlugin {
         "diffeditordocument.h",
         "diffeditorfactory.cpp",
         "diffeditorfactory.h",
-        "diffeditorguicontroller.cpp",
-        "diffeditorguicontroller.h",
         "diffeditormanager.cpp",
         "diffeditormanager.h",
         "diffeditorplugin.cpp",
         "diffeditorplugin.h",
-        "diffeditorreloader.cpp",
-        "diffeditorreloader.h",
         "differ.cpp",
         "differ.h",
         "diffutils.cpp",