[closes #13] enhanced observer mode additions and cleanup (#45)

* additions and cleanup [skip ci]

* disallow observers to change their faction and team [skip ci]

* disable observer slots from admin control

* when checkboxObserversAllowed is toggled, update correctly

* Also adjusted the labels better

* when scenario is checked, observer mode is disabled and extra slots are
closed

* attempted to add a feature that would allow the admin to change to a
different slot (doesn't work yet)

* graphics still needs some adjustment

* host can not go to a slot > mapInfo.maxPlayers

source/shared_lib/sources/map/map_preview.cpp

@@ -1107,6 +1107,10 @@ bool MapPreview::loadMapInfo(string file, MapInfo *mapInfo, string i18nMaxMapPla
 				mapInfo->size.y	= header.height;
 				mapInfo->players= header.maxFactions;
 
+				// hardMaxPlayers is used in menu_state_custom_game, as part of
+				// enhanced observer mode (issue #13)
+				mapInfo->hardMaxPlayers = mapInfo->players;
+
 				mapInfo->desc 	=  i18nMaxMapPlayersTitle 	+ ": " + intToStr(mapInfo->players) + "\n";
 				mapInfo->desc 	+= i18nMapSizeTitle 		+ ": " + intToStr(mapInfo->size.x) + " x " + intToStr(mapInfo->size.y);