Allow adding "directory imports" in QDS

In QDS 2.0 it is possible to add so-called directory imports:

In this case "CP 2Horizotnal Layout" is an import that "imports" a sub folder.

This import will make all types in the subfolder available.

In QDS 2.1 this kind of import cannot be added using the UI.

Directory imports are crucial for large projects and the standard Qt Bridge workflow.

As far as I can see direcotry imports were not handled explictly in QDS 2.0:

m_addImportComboBox->addItem(possibleImport.toString(true), QVariant::fromValue(possibleImport));

https://github.com/qt-creator/qt-creator/blob/4.14/src/plugins/qmldesigner/components/importmanager/importswidget.cpp

The distinction is built in into QmlDesigner::Import:

 bool isFileImport() const { return m_url.isEmpty() && !m_file.isEmpty(); }
 bool isLibraryImport() const { return !m_url.isEmpty() && m_file.isEmpty(); }

Most likely the issue is in the frontend and the new code relies on an existing url, while directory imports only have file.