[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/include/map/map_preview.h

@@ -99,11 +99,13 @@ public:
 
 	Vec2i size;
 	int players;
+	int hardMaxPlayers;
 	string desc;
 
 	MapInfo() {
 		size 	= Vec2i(0,0);
 		players = 0;
+		hardMaxPlayers = 0;
 		desc 	= "";
 	}
 };