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.pro

@@ -7,10 +7,8 @@ HEADERS += diffeditor_global.h \
         diffeditorcontroller.h \
         diffeditordocument.h \
         diffeditorfactory.h \
-        diffeditorguicontroller.h \
         diffeditormanager.h \
         diffeditorplugin.h \
-        diffeditorreloader.h \
         differ.h \
         diffutils.h \
         diffview.h \
@@ -22,10 +20,8 @@ SOURCES += diffeditor.cpp \
         diffeditorcontroller.cpp \
         diffeditordocument.cpp \
         diffeditorfactory.cpp \
-        diffeditorguicontroller.cpp \
         diffeditormanager.cpp \
         diffeditorplugin.cpp \
-        diffeditorreloader.cpp \
         differ.cpp \
         diffutils.cpp \
         diffview.cpp \