Allow to keep contents when deleting a combined fragment

[Matthias Schoettle]

When the user deletes a combined fragment, the user might want to keep the contents. Instead of deleting the contents automatically, the user should get the choice of whether the contents should be kept or deleted.

[Matthias Schoettle]

Refactor: Changes handling of adding/removing of combined fragments, operands and their contents.

When a combined fragment is added or removed, all its contents are explicitly added or removed as well. The fragment views are added or destroyed by the MessageViewView. I.e., destroying is not handled by the LifelineView anymore.

This is more consistent across initial build up and handling of events. Also, it facilitates further improvements.

Related issues:

refs #165 refs #470

→ <<cset ad482df9fb2d>>

[Matthias Schoettle]

Resolves #470: Adds option to delete combined fragment but keep contents.

The contents of the combined fragment are moved to the same index the combined fragment was located at.

Also adds test cases.

→ <<cset 06ede10ca065>>