Fix #7: not that smooth but working at least.
Display current relations with selected empire in selection menu.
Permit to change status with selected empire in selection menu.
Know all possible diplomacy states only if "startWithDiplomacy" custom
option is selected.
Clearer list to select from in SelectBox objects.
Keep "startWithDiplomacy" setting value when restarting a game.
No reason to start AI before the first turn.
AI try to be at war with only one enemy.
Do not let AI declare war or peace to itself.
AI do not declare war AND sign treaty with same entity.
When an empire surrender, it must provide atribute, whereas its opponent
has a treaty. The tribute can only be stopped by declaring a war.
Clearer code when AI makes diplomatic proposals.
Correct bug when applying diplomatic proposals.
Add helper method to get all the empires we have some relation with.
When involved in multiple wars, try to have only one.
Implement tribute as power advancement transfered from the tributary to
the overlord. It is a part of #9.
Do not influence tiles when having a treaty. Part of #9.
Add utility method to get the entity which has the main influence on a
tile.
Fix bug when finding neighboring empires.
Avoid NPE when applying treaties.