Add sack button to soldier info screen

Currently a player must look at a soldier's stats, decide a sacking is
in order, click ok, click ok, click sell/sack, forget the name of the
soldier, go back to the soldier's list, back to sack/sell, can't find
the soldier, more clicks, remove soldier from craft, yet more clicks,
finally sack the soldier.
This extra button might lessen player stress.
Correct keyboard action

Cancel key cancels sacking, ok key proceeds with sacking.
Merge pull request #548 from cfailde/sacking

Add sack button to soldier info screen