Usability improvements for creation and deletion messages

The current state allows users to create creation and deletion messages only as the first or the last element on a lifeline (receive ends).
This will result in shifting the lifeline down/ending the lifeline accordingly.
If we loosen the requirements to create those messages (first/last), we need some kind of post-command mechanism which checks the diagram if a creation or deletion has become the first/last one through any action (e.g. a move of a lifeline or the deletion of the previously first/last element).
When those messages are not first or last one, they shall behave like any other message.

Also see issue #46 for more information on the creation message use case.