Creation preview improvements

[hlxid]

• Fix temporary element cleanup when canceling a creation.
  This is important for the edge creator. Previously the creation was cancelled by undoing the last action which is the element insertion. However for edges this may not be the case as the viewport can be moved around which also is a command and would be undone instead of the insertion. To fix this the element now is only inserted using the AddElementToGraphAction when the creation is finalized.
• Only insert the preview edge into the diagram once the source has been determined. Previously it was inserted with the enablement of the edge creation tool. But because the source and target are unset in this situation sprotty can't render it and will show a ? at the diagram origin. Now it is only inserted once the source is known and the temporary target element is inserted