Refactored verification of land claim permissions

 - New class: FClaimChecker. Responsible for verifying that land claim is allowed.
 - New class: BaseClaimChecker. Implements only failure conditions. Intended for use by replacements to DefaultClaimChecker.
 - New class: DefaultClaimChecker. Implements default behavior of Factions.
 - New config option: claimingFromOthersNotifies (default: true). If true, will notify online members of enemy faction when you claim their territory.
 - Sending message to player moved to FPlayer.attemptClaim()
 - Current claim checker is stored in P.class
 - Function to set current claim checker in P