Core: Remove #include <QMainWindow> from icore.h

Adapt users; also use dialogParent() instead of mainWindow() were
appropriate.

Change-Id: Ib60b118f05c986a70657446c5b2937074243bb5c
Reviewed-by: hjk <hjk@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/plugins/coreplugin/icore.h

@@ -32,7 +32,6 @@
 #include <utils/qtcsettings.h>
 
 #include <QList>
-#include <QMainWindow>
 #include <QObject>
 #include <QRect>
 #include <QSettings>
@@ -40,6 +39,7 @@
 #include <functional>
 
 QT_BEGIN_NAMESPACE
+class QMainWindow;
 class QPrinter;
 class QStatusBar;
 class QWidget;