DesignMode: Apply 'static pattern'

Also:
 - and replace some occurrences of DesignMode::instance()->id() by
   Core::Constants::MODE_DESIGN for less dependence on the lifetime
   of the DesignMode object (and less indirection)
 - remove storage if DesignMode::instance() values when direct
   use of the static functions suffice
 - remove some unused items from the interface
 - use member-initialization in DesignMode::Private.

Change-Id: Ie66c06da0fc0a3ccc588b8079e51db6b39284152
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/designmode.h

@@ -30,8 +30,6 @@
 namespace Core {
 class IEditor;
 
-namespace Internal { class DesignModeCoreListener; }
-
 /**
   * A global mode for Design pane - used by Bauhaus (QML Designer) and
   * Qt Designer. Other plugins can register themselves by registerDesignWidget()
@@ -39,8 +37,6 @@ namespace Internal { class DesignModeCoreListener; }
   * to the main editor widget itself.
   */
 
-class DesignModePrivate;
-
 class CORE_EXPORT DesignMode : public IMode
 {
     Q_OBJECT
@@ -51,15 +47,13 @@ public:
 
     static DesignMode *instance();
 
-    void setDesignModeIsRequired();
-    bool designModeIsRequired() const;
+    static void setDesignModeIsRequired();
+    static bool designModeIsRequired();
 
-    void registerDesignWidget(QWidget *widget,
-                              const QStringList &mimeTypes,
-                              const Context &context);
-    void unregisterDesignWidget(QWidget *widget);
-
-    QStringList registeredMimeTypes() const;
+    static void registerDesignWidget(QWidget *widget,
+                                     const QStringList &mimeTypes,
+                                     const Context &context);
+    static void unregisterDesignWidget(QWidget *widget);
 
 signals:
     void actionsUpdated(Core::IEditor *editor);
@@ -70,9 +64,6 @@ private:
     void currentEditorChanged(IEditor *editor);
     void updateContext(Id newMode, Id oldMode);
     void setActiveContext(const Context &context);
-
-    DesignModePrivate *d;
-    friend class Internal::DesignModeCoreListener;
 };
 
 } // namespace Core