Make gui2::event::message noncopyable

This makes it impossible to accidentally pass a message by value, which
avoids problems like the crashes @Vultraz got when refactoring the event
dispatcher.

src/gui/core/event/dispatcher.cpp

@@ -174,8 +174,7 @@ bool dispatcher::fire(const ui_event event, widget& target, void*)
 bool dispatcher::fire(const ui_event event, widget& target, message& msg)
 {
 	assert(find<set_event_message>(event, event_in_set()));
-	// NOTE: std::ref() needed here to prevent reference decay.
-	return fire_event<signal_message_function>(event, this, &target, std::ref(msg));
+	return fire_event<signal_message_function>(event, this, &target, msg);
 }
 
 void dispatcher::register_hotkey(const hotkey::HOTKEY_COMMAND id, const thotkey_function& function)

src/gui/core/event/message.hpp

@@ -46,6 +46,11 @@ namespace event
  */
 struct message
 {
+	message() = default;
+
+	// Disallow copying because constructing a copy loses the exact type.
+	message(const message&) = delete;
+
 	virtual ~message()
 	{
 	}