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/scenedit/scen.fileio.cpp

@@ -245,6 +245,7 @@ void writeScenarioToXml(ticpp::Printer&& data, cScenario& scenario) {
 				case eShopItemType::OPT_ITEM:
 					quantity %= 1000;
 					chance = entry.quantity / 1000;
+					BOOST_FALLTHROUGH;
 				case eShopItemType::ITEM:
 					data.OpenElement("item");
 					if(quantity == 0)