Added a new hook for class Alias, function toggleActive, hook preToggle which can be used to stop the toggle function with a hook for class Alias.
A not existing hook function in a plugin will always cause a true return value to not break interrupt flow.
Make sure the toggle function for the alias return the correct value to return error message to user why a alias could not be deactivated.
Added method to check if it is allowed to deactivate an alias.