Add option to set default variant in variant set

Figma always stores default variant of component set, but importer doesn't expect this type of metadata. Default variant is not 0-th/n-th element of the children-list, instead it's the most top left variant of visual component set organization.

We should be able to designate default component on the ArtboardSet level. It should be translated to generated qml, by assigning proper default state value to state: "..." property. We can export default node itself, it's uuid or index in ArtboardSet.