To make room, add ability to self-destruct buildings

[stefanhendriks]

• Ie, select button to self-destruct.
• click building (mark for self-destruct)
• building has timer (5 seconds?) (grace period)
• click again to stop it, or do nothing and it will self-destruct. Exploding, leaving rubble, etc.

• this is instead of selling, which is too much C&C like (and does not make sense in a Duniverse)