Allow to lock phone number of detonator box

[Wuzzy2]

I suggest to add a new property to detonator boxes: PhoneNumberLocked. This can be true or false and is false by default.

If true, the phone number of the detonator box can not be edited by the user, only the phone number as specified in PhoneNumber property is used. If false, the phone number of the detonator box can be chosen freely (current behaviour).

Special case: If PhoneNumberLocked is true and PhoneNumber is unset or empty, the detonator box is “broken”, it won't make any phone calls, ever (might be reflected in tooltip).

Use case for locking the phone number:

• Disable number selection for early levels with dynamite / detonator boxes
• Interesting new puzzle designs