Layout parts split in different sections

[Bramjetten]

Add support for different sections for layout parts like so:

Instead of assigning an array to theme.layout_parts, it's also possible to assign a hash:

theme.layout_parts = {
  brand: %w[logo brand_color],
  google_reviews: %w[google_reviews google_rating],
  docs: %w[terms_and_conditions privacy]
}

My proposal is to use I18n to add labels for different sections under the spina.layout.sections key.