Eliminate use of __attribute__

Ideally this would be standard C++, but here I've settled for things that should be supported by both clang and VS/cl.exe:
- Deprecated attribute retained, but now uses __declspec syntax
- Packed attribute replaced with pragma pack, except one instance where it unnecessary
- Aligned attribute replaced with explicit padding bytes inserted in the structs where needed
- Unused attribute simply removed (though where possible, the unused entities were also removed)

osx/dialogxml/message.cpp

@@ -18,7 +18,7 @@ void cTextMsg::attachClickHandler(click_callback_t f) throw(){
 	clickable = onClick != NULL;
 }
 
-void cTextMsg::attachFocusHandler(focus_callback_t f __attribute__((unused))) throw(xHandlerNotSupported){
+void cTextMsg::attachFocusHandler(focus_callback_t f) throw(xHandlerNotSupported){
 	throw xHandlerNotSupported(true);
 }