Insertion / deletion of new elements

[kulla]

• Insertion of elements via / inside ProseMirror
  • When ProseMirror elements (Heading, etc.) are used: Insert into current prosemirror instance + only allowed elements can be inserted
  • When complex elements (= interactive exercise) is inserted => ProseMirror-Element is split
• Insertion of elements via toolbar button
• Deletion of elements
• Handle case of zero elements