improve generated code style

Currently we generate one line per property for each property added. So we end up with code like this:

Button {
    baseProperties.opacity: 1
    normalProperties.bevel: true
    baseProperties.radius: 20
    ...
}

Which can get very messy when you generate a lot of properties. We should group the property sets together in the code to improve readability for Developers.

Button {
    baseProperties {
        opacity: 1
        radius: 20
    }
    normalProperties.bevel: true
    ...
}