Fix toplevel in Qt Designer plugin.

The toplevel is used as a parent for Designer's dialogs and for dragging.
Currently, when using Qt 5, for example the dialog about missing resource
files shows up at 0,0 and and Accessibility on Windows emits warnings about
incorrect hierarchy since the Editor manager is not visible in Design mode.

Change-Id: I49e77db89bf93e8342beb3e4ace764a14d0231f1
Reviewed-by: Jarek Kobus <jaroslaw.kobus@digia.com>

src/plugins/designer/formeditorw.cpp

@@ -150,7 +150,7 @@ FormEditorW::FormEditorW() :
     qFill(m_designerSubWindows, m_designerSubWindows + Designer::Constants::DesignerSubWindowCount,
           static_cast<QWidget *>(0));
 
-    m_formeditor->setTopLevel(Core::ICore::editorManager());
+    m_formeditor->setTopLevel(Core::ICore::mainWindow());
     m_formeditor->setSettingsManager(new SettingsManager());
 
 #if QT_VERSION >= 0x050000