Core: Pimpl MainWindow

I'd like to disentangle things a bit, and there's quite a bit
non-window-ish stuff here.

Change-Id: Iad1e1d17ccd10b3ffff14251039ae2a5d6bfb675
Reviewed-by: Eike Ziller <eike.ziller@qt.io>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

src/plugins/coreplugin/icore.h

@@ -29,7 +29,10 @@ namespace Core {
 class Context;
 class IWizardFactory;
 
-namespace Internal { class MainWindow; }
+namespace Internal {
+class MainWindow;
+class MainWindowPrivate;
+} // Internal
 
 class NewDialog;
 
@@ -38,6 +41,7 @@ class CORE_EXPORT ICore : public QObject
     Q_OBJECT
 
     friend class Internal::MainWindow;
+    friend class Internal::MainWindowPrivate;
     friend class IWizardFactory;
 
     explicit ICore(Internal::MainWindow *mw);