Strife's slow states

In Strife's easiest difficulty setting, some monsters act slowly, making them quite easier to fight. This is achieved in a similar way to how the demons and spectres are faster in nightmare mode in Doom, by changing the tic length of certain states (doubled instead of halved as in nightmare).

This pull request implements that with a new state flag, "slow", and flags the appropriate states. Thanks to Quasar for mentioning the issue in Eternity's changelog and for allowing me to use the Chocolate Strife code as reference to identify which states needed the flag.