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.

proj/xc12/BoE.xcodeproj/project.pbxproj

@@ -2348,6 +2348,7 @@
 					"-Wno-quoted-include-in-framework-header",
 					"-Wno-shorten-64-to-32",
 					"-Wno-comma",
+					"-Werror=implicit-fallthrough",
 				);
 			};
 			name = Debug;
@@ -2455,6 +2456,7 @@
 					"-Wno-quoted-include-in-framework-header",
 					"-Wno-shorten-64-to-32",
 					"-Wno-comma",
+					"-Werror=implicit-fallthrough",
 				);
 			};
 			name = Release;