Add a new var to cache: OnGridKillableNPCs, so we can determine from combat if we should be running GetBestTarget() yet. We should only run that routine when their are entities targeted and available to kill.

Adjusts GetBestTarget to priorities within primaryweapons priority targets and drone priority targets better, If their is a HIGHER priority target avail then the one we are shooting at it should switch to the high priority target. (we may need to fix the targeting and untargeting routines to do the same!)