Implement support for "display" and "visibility" attributes in SVG Generators

There are two attributes that controls if an svg element will be rendered or not. They look similar but they serve different purposes. If "visibility" attribute is set to "hidden", the node and all of its children are not rendered. However, if any of the children set the "visibility" to something else, this child will be shown. This is not the case for "display" attribute, where the children will not be rendered even if any of the children tried to override the value.

The scenario above applies to all nodes including the <svg> element. In our case, we need to create an empty root item that will be empty so that we can have a valid QML file. Additionally, this will be the same for the QuickItem generator, where we will have an empty QQuickItem.