Automatically fill prefix of a SourcePart inserted in an existing summary

## Feature
### User Story
As a user when I insert a sourcepart in the middle of a summary i want to have the choice to automatically fill in the prefix of the new sourcepart to conform with the summary or manually change it.

### Description
It is currently only possible to manually change the prefix of sourceparts inside a summary. To achive this feature a new functionality should be added which recognizes the pattern of the prefix of the summary and autofills it.