Add reply message for undefined constructor

[Matthias Schoettle]

Currently, when the behaviour of a constructor is undefined, no reply message is created when allowing to define its behaviour within another message view (nested behaviour).

However, this causes lots of problem when dealing with removal of messages etc.

[Matthias Schoettle]

References #479: Creates reply messages for create messages with no behaviour.

→ <<cset 91a1b87f58af>>

[Matthias Schoettle]

Resolves #479: Adds UI support for create messages with replies.

Adds the fragment to the spacer the spacer is coming after. This allows to easier retrieve the fragment before the spacer and supports the reply messages for create messages properly.

→ <<cset 036c478ed0b9>>

[Matthias Schoettle]

References #479: Fixes whether message creation is allowed.

• The receive event side is now always the opposite of the send event side.
• Considers partiality of signature to ensure only non-partial operations might allow nested behaviour
• receive event after create message does not always allow message creation now

→ <<cset 06fc146d9dba>>