Move Command CommonGuards to required interface property

[DavidRieman]

It's weird that every Action has to have a Guards section but then also always calls VerifyCommonGuards. VerifyCommonGuards feels like something that should happen for us automatically before even Guards is called. This could happen if the ActionGuards list was something that the GameAction class required us to provide publicly on the derived classes.

(It's also questionable whether we should just collapse Guards and Execute, or whether their separation continues to have some value. It's hard to decide what to guard from in Guards and what to defer to Execute, and it usually means making the class hold some data temporarily between the two calls which is not exactly ideal. Consider further exploration / discussion / logging new issue, after making this Guards improvement)

(Relates to #63)