More menu items can be disabled.
Disable "Destroy" for deleted machines and containers.
Disable add container more menu on deleted machines.
Don't place units on deleted machines.
Merge pull request #525 from jaycee/do-not-use-deleted-machines

Do not allow units or containers to be added to deleted machines.

* Disable menu items that make no sense for deleted machines (e.g. Add
  container, Destroy)
* Cancel unit placement on machines that are deleted, with notification as to
  why it failed.