Layout blocks widgets

As a Studio app builder, I can drag and drop widgets to reuse my layout blocks

AC
Following widgets are available in the "Layout Blocks" category

• 1 widget for each layout block configured, ordered alphabetically

• When I drag and drop a layout block, it is bound to:
  • doctype layout: current document
  • document tab: current document

• Analytics event is sent when d&d a widget
  • Category: Described according to NXS-5742
  • Event: Widget drag and drop
  • Label: Layout block
    (label should be kept generic as it would contain project specific information)