Show reason to user when validity check of input failed

[Matthias Schoettle]

Currently, whenever the user types in something (like the attribute or operation signature, changes class or operation names) the keyboard is not getting dismissed until a valid input is provided.

Issue #75 might help a little bit, but it might still be confusing if there is just a minor mistake or it is related to a name not being unique.

Therefore, it would be good to have proper feedback to the user to inform what exactly is wrong. Not sure if a message window in a corner is good. It might be better to show it exactly in that location.