Fixed an issue with the new CB flickering on and off.

• Added new Casus Belli - Border Clash. Two opposing empires with bordering systems can make claims on those bordering systems and then go to war for them.
  
• In a Border Clash war the attacker's war exhaustion rate is halved, the defender's war-exhaustion rate is doubled.
  
• Game-rule can_add_claim is now over-ridden.
  
• Added triggered mid to late-game strong negative opinion modifiers for fanatics towards opposing ethics and/or passive empires, to make fanatics more likely to war.
  
• Added triggered negative opinion modifiers for empires with resource shortages, towards opposing neighbours with surplus of the needed resource.
  
• Increased AI aggressivenss multiplier when boxed in, in defines, from 4 to 8.