ZScriptify a detection check in P_LookForPlayers

[mykola-ambar]

This PR zscriptifies a detection check in P_LookForPlayers, and adds two new functions to Actor: virtual bool CanDetect(Actor other) and virtual bool CanBeDetectedBy(Actor other). Both must return true for the check to pass.

[coelckers]

Three things:

1. Please do not remove comments in unrelated code.
2. The way you set the static FName variables is VERY inefficient and involves a mutex! Please change this to constants to be defined in namedef_custom.h. With this function I am not even sure that fully scriptifying it would be better because it also saves the dynamic lookup of the two variables.
3. Making CanDetect native is pointless. The VM has a shortcut for empty functions that is a lot faster than calling into an empty native function. So this should be moved into script code as well.