Added beacon.

This construct generates quit a lot of warnings. How about converting it into a per-effect switch?
```cpp
switch (a_Effect)
{
  case cEntityEffect::effRegeneration: return (a_BeaconLevel >= 4);
  case cEntityEffect::effStrength:     return (a_BeaconLevel >= 3);
  ...
  case cEntityEffect::effNoEffect: return true;
  default:
  {
    LOGWARNING("%s: Invalid beacon effect: %d", __FUNCTION__, (int)a_Effect);
    return false;
  }
}
```