[Feature] Improve lobby interface: Drag-and-drop team assignation

[albertz]

Converted from SourceForge issue 3290829, submitted by nubsis

For a team game, players are currently assigned to teams by pressing the team icon by the player. This works and looks horrible. If each team had a "drop area" where the host could drop players to assign them to the team the process would be more intuitive and look better. This would leverage the other related feature request on lobby interface improvements.