battleCalculator #1 (fix issue 12688 battle calc attacker/defender determination)

[frigoref]

issue 12688 (battle calc on own territory shows own player as attacker and defender) Desired:

1. Attacker: Current player always, except when only allied units present (then next enemy)
2. Defender: As enemy to attacker with most units (fallback: whomever is at war and has the next turn)

AttackerAndDefenderSelectorTest.java

• simplification of method getAttackerAndDefender()
• new methods getBestAttacker() and getBestAttacker()
• removed now unused methods getOpponentWithPriorityList, playersAtWarWith, neutralPlayersTowards, getEnemyWithMostUnits

UnitCollection.java

• rename method getPlayersByUnitCount to getPlayersSortedByUnitCount

BattleCalculatorDialog.java

• extract common logic from method addAttackers and addDefenders to new method adjustBattleCalculatorPanel
• extract static attribute modifications from instance method dispose into new static method disposeInstance (especially pack also for adding attacker)

BattleCalculatorPanel.java

• introduce new methods setAttackerWithUnits and setDefenderWithUnits to limit attacker/defender change to own instance
• make setter of attribute attacker/defender private
• rename methods hasAttackingUnitsAdded and hasDefendingUnitsAdded to hasAttackingUnits and hasDefendingUnits
• rename attribute attackerUnitsTotalHitpoints to attackerUnitsTotalHitPoints

Notes to Reviewer

[frigoref]

Should solve to issue 12688 (2.6: When opening battle calc on own territory on own turn, your country is selected as both attacker and defender)