Refactor uiGroupHandlers

[sprunk]

Right now this is an array of group handlers indexed by teamID. There's two broad users, one is the skirmish AI interface which indexes it with given AI's team, and the rest is most of the player-facing UI which indexes it with gu->myTeam.

Change this into one standalone grouphandler for the player UI (everywhere it's gu->myTeam currently) and have the array indexed by AI ID. This way player units won't lose group when changing teams (capture, dynamic comshare) and players/AIs that happen to be on the same team won't interfere with each other's groups.

Somewhat related: #1099