#3497648: Focus mode for global regions

• Update routing and uiSlice state to handle selected region
• Deselect currently selected component when switching region
• Allow selecting a region from the UI
• Restrict SortableJS to work only within selected region
  • Preview canvas
  • Layers
• UI changes for focused region — when it's not Content
  • Preview canvas visual changes (darken other regions)
  • Top bar changes with action to exit from focus
• UI overhaul for the Layers panel
  • Renamed in codebase to layers (from tree)
  • Updated look and feel
  • Refactored to have separate React components for regions, components and slots.
• Existing Global regions e2e tests are broken - but probably need to basically be re-written
• Media library e2e test is also being stubborn - it's possible that I've broken this but struggling to track down the problem
• Need some unit/component tests for the new functionality

There are some minor bugs I've spotted

• Drag and drop in Layers is tricky to drop into bottom position
• Drag and drop in Layers is flickering when trying to drop into empty slot
• When focused into a Region (other than Content) drag and drop in the preview fails and breaks the page if you release the mouse while the cursor is over the greyed out background

I think these should be addressed in follow up tickets which I'll create when this is merged.