Use dialogParent() instead of mainWindow()

There are very few reasons to use mainWindow() directly.
Especially for modal dialogs, using dialogParent() is important, since
that guarantees the stacking order in case of other dialogs currently
being open.

Change-Id: I7ad2c23c5034b43195eb35cfe405932a7ea003e6
Reviewed-by: hjk <hjk@qt.io>

src/plugins/debugger/debuggerruncontrol.cpp

@@ -709,7 +709,7 @@ void DebuggerRunTool::start()
 
             static bool checked = true;
             if (checked)
-                CheckableMessageBox::information(Core::ICore::mainWindow(),
+                CheckableMessageBox::information(Core::ICore::dialogParent(),
                                                  tr("Debugger"),
                                                  warningMessage,
                                                  tr("&Show this message again."),