Provide better support to ensure consistency between views

[Matthias Schoettle]

Right now, when message views (probably applies to state views as well) are created and the structure is modified after, this can lead to inconsistency (e.g., removing an operation).

The tool should provide proper feedback to the user and not allow to save until they are resolved. Otherwise, invalid models are the result, which can lead to problems when loading the models again.

One way would be to use the validation, perhaps in combination with a split view so that it is easier to see things right away as they happen.

[Matthias Schoettle]

References #267: Prevents a problem if an operation is deleted and after, a message view where this operation was called is loaded for the first time.

→ <<cset 6af077cbb085>>

[Matthias Schoettle]

Accidentally deleted this comment:

References #267: Prevents an exception to occur if a message has no signature (due to an incorrect model).

→ <<cset 4b27cbbe5943>>