Make the menu close {add,cred,del}Box when it's opened
Add 'memory' to CredDialog and fix its buttons' reactivity.

Don't fall back to the username and password stored in the schema for the currently processed device every time an authentication error arises: use instead the last
user-provided ones.
Fix 'Execute' button's reactivity: after an authentication error arises, don't make the button reactive till at least one of the username/password fields gets changed.
Move a handful of support functions from extension.js to utilities.js