DesignMode/Qt Designer clean-up Part V: Renaming

Re-introduce FormWindowEditor as IEditor used for
Designer. Use SharedTools::WidgetHost directly
for embedding into FormEditorStack, doing the
geometry-changed handling in there as well.

src/plugins/designer/formeditorfactory.cpp

@@ -74,7 +74,7 @@ Core::IFile *FormEditorFactory::open(const QString &fileName)
 Core::IEditor *FormEditorFactory::createEditor(QWidget *parent)
 {
     const EditorData data = FormEditorW::instance()->createEditor(parent);
-    return data.xmlEditor;
+    return data.formWindowEditor;
 }
 
 QStringList FormEditorFactory::mimeTypes() const