refactor: Element custom views, collaboration improvements

[areknawo]

• Change element custom view handling
  • Only add structure on init;
  • Cursor navigation changes (block gap cursor, node selection inside custom view);
  • Ability to switch between custom and raw view;
  • New side menu;
• Improved collaboration:
  • Maintain node selection on sync;
  • Collaborative gap cursor;
• Raw element view
  • Improved editor handling;
  • Remove element when no tag;