Design mode integration

Moved Qt Designer to open from Design Mode. Also, Design mode is now global,
and created in coreplugin. Other plugins can register themselves to it.

src/plugins/coreplugin/coreplugin.h

@@ -33,6 +33,7 @@
 #include <extensionsystem/iplugin.h>
 
 namespace Core {
+class DesignMode;
 namespace Internal {
 
 class EditMode;
@@ -59,6 +60,7 @@ private:
 
     MainWindow *m_mainWindow;
     EditMode *m_editMode;
+    DesignMode *m_designMode;
 };
 
 } // namespace Internal