Show the selection panels to observers / in replays, fixes #3731.

This allows observers to see the training queue, researched techs, unit stance, constructable buildings, garrisoned units, market prices etc. for all players without changing the perspective.
All buttons are disabled if the user isn't allowed to control the affected player.
Notice updateUnitCommands() fills out the panel differently in case of selecting an allied building.
Extends the GUIInterface methods "GetNeededResources", "CheckTechnologyRequirements" and "IsTechnologyResearched" to optionally take a different player-argument.