Implement Ctrl-D: Self-Destruct selected units (toggle on/off countdown)

[KevinHake]

If units are selected, Ctrl-d begins a self-destruct countdown. Repeat Ctrl-d to cancel the countdown for all units selected (after which Ctrl-d will again start at the top of the countdown).

Things to research:

• Pretty sure self destruct explosions are different than just getting killed
• What's the countdown? How many ticks between them?
• In game text notifications:
  • Do we need to open a dependent issue for this? Not sure it exists yet
  • What are the specific messages that need to be sent to the UI screen? (compare w/ OTA)
• Edge case: What should happen for the following sequence of events:
  1. I select units A, B, and C and then start the self destruct
  2. I de-select unit A, select unit D, then again hit Ctrl-D Question:
     • Do unit B and C terminate their self-destruct sequence while unit D begins the sequence (meanwhile we expect A to continue the countdown)?
     • Or, is the behavior that Ctrl-D only begins self-destruct if no units in the current selection are currently counting down, and thus only issues the cancel command if some fraction of the selection is already in the countdown?