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
fixes #2190 added announcer voice upon game start

Thanks to @Nyerguds for mixing this together for the RA mod!
add a close button with hotkey to diplomacy widget
Merge pull request #2819 from Mailaender/game-start-sound

Battlecontrol initialized!
Merge pull request #2818 from Mailaender/diplomacy-renovation

Bring the Diplomacy menu back to life...
Fix compile error for people running ancient C# 3 compilers