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.

[mykola-ambar]

Regarding 2: thanks for letting me know - I'm not as familiar with inner workings of those parts of GZDoom, so I didn't realise that FName had so much going under the hood - I thought it was a trivial data type that wouldn't have mutexes or other side effects. I'll rework it.

Question regarding 3: what do you mean by an empty function? As in, I can see how a void function can be empty, but this function has to return true - will a simple return true; count as empty in this case? I tried making an empty virtual bool TestBool() {} function in ZScript, and GZDoom violently crashed while loading ZScript, so I assume that's not what you meant. I need some clarification before I push something useless. Thanks in advance!

[RicardoLuis0]

will a simple return true; count as empty in this case?

yes

[mykola-ambar]

Thanks @RicardoLuis0!