Core: return QMainWindow * from Core::mainWindow

... instead of QWidget *

Change-Id: Ibe38814ae26524a9ffa4c14d170d5aa487a40931
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/coreplugin/icore.cpp

@@ -486,7 +486,7 @@ QWidget *ICore::currentContextWidget()
 }
 
 
-QWidget *ICore::mainWindow()
+QMainWindow *ICore::mainWindow()
 {
     return m_mainwindow;
 }