Better support the QtDesignStudio workflow

When opening a project with QtDesignStudio all autogenerated APIs created with the Interfaceframework are not available.

This can fixed by adding a generator template, which generates only a qml-mock.
this mock can be generated on every IDL change and put into source-control.

Using the mock makes it possible to use the project in QtDesignStudio, but use a real autogenerated code for the actual production code.