Use Core::Id in ModeManager interface

Makes it more uniform to use and allows placeholder widget
creation to be independent of mode creations.

Change-Id: I4021bc9db7f8c78f0374c0cc3b3331506959afe4
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/plugins/designer/formeditorstack.h

@@ -27,6 +27,8 @@
 
 #include "editordata.h"
 
+#include <coreplugin/id.h>
+
 #include <QStackedWidget>
 #include <QList>
 
@@ -35,10 +37,7 @@ class QDesignerFormWindowInterface;
 class QDesignerFormEditorInterface;
 QT_END_NAMESPACE
 
-namespace Core {
-    class IEditor;
-    class IMode;
-}
+namespace Core { class IEditor; }
 
 namespace Designer {
 namespace Internal {
@@ -68,7 +67,7 @@ public slots:
 
 private slots:
     void updateFormWindowSelectionHandles();
-    void modeAboutToChange(Core::IMode *);
+    void modeAboutToChange(Core::Id mode);
     void formSizeChanged(int w, int h);
 
 private: