Make implicit switch fallthrough an error in the Xcode project and fix all places that do it

Since we can't directly use C++11 attributes, I've chosen BOOST_FALLTHROUGH to annotate intentional fallthroughs.

Thanks to @fosnola for spotting a missing break, which triggered this.

src/scenario/item.cpp

@@ -311,6 +311,7 @@ cItem::cItem(eItemPreset preset) : cItem() {
 		case ITEM_SPECIAL:
 			item_level = -1;
 			full_name = "Call Special Node";
+			BOOST_FALLTHROUGH;
 		case ITEM_SHOP:
 			graphic_num = 105; // The blank graphic
 			break;