VectorImage/svgtoqml: No path container handling on root level

When the svgtoqml tool was refactored, the visitStructureNodeStart() was given a special case for the root node: visitDocumentNodeStart(). However, the logic to handle path containers (so that multiple paths can be combined in a single shape item) was never copied into the root node functions, hence making it impossible for the root node to be a path container.

This leads to suboptimal handling of some SVG files, where every path under the root is given its own separate Shape item: https://codereview.qt-project.org/c/qt/qtdeclarative/+/571193/3

The logic to handle path containers should be added to the root node handling as well.