Move Gui stuff out of DiffEditorController

Place it in DiffEditorGuiController, will be needed for
sharing gui settings between unified and sidebyside versions.

Change-Id: I8a858d0549ff84e21f7c909288ca07c5a2ef4e5b
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>

src/plugins/diffeditor/diffeditor.pro

@@ -7,6 +7,7 @@ HEADERS += diffeditor_global.h \
         diffeditorcontroller.h \
         diffeditordocument.h \
         diffeditorfactory.h \
+        diffeditorguicontroller.h \
         diffeditormanager.h \
         diffeditorplugin.h \
         differ.h \
@@ -16,6 +17,7 @@ SOURCES += diffeditor.cpp \
         diffeditorcontroller.cpp \
         diffeditordocument.cpp \
         diffeditorfactory.cpp \
+        diffeditorguicontroller.cpp \
         diffeditormanager.cpp \
         diffeditorplugin.cpp \
         differ.cpp \