bring the Diplomacy menu back to life, but

- rename LockTeams to FragileAlliances to avoid confusion
- only allow it in Free-For-All, not Team games or Missions
- grey out non-functional buttons and disabled checkboxes
add a close button with hotkey to diplomacy widget
Merge pull request #2818 from Mailaender/diplomacy-renovation

Bring the Diplomacy menu back to life...