Improve handling of block separators

From old feedback: Improve handling of block separators | Cwicly

The separators are basically great, but the following points would still be helpful:

  • Integrate own SVG code (maybe globally in the plugin settings).
  • Show a preview in the selection list or allow a quick scroll with the arrow keys, each of which updates the page preview
  • Take over the background color of the preceding or following block (depending on position) automatically or by manual selection, since the separators should be visually transitionless.