BreakpointManager: Safer and simpler updates.

Windows/MainWindowMenu.cpp

@@ -370,6 +370,7 @@ namespace MainWindow {
 	static void UmdSwitchAction(RequesterToken token) {
 		auto mm = GetI18NCategory(I18NCat::MAINMENU);
 		System_BrowseForFile(token, mm->T("Switch UMD"), BrowseFileType::BOOTABLE, [](const std::string &value, int) {
+			// This is safe because the callback runs on the emu thread.
 			__UmdReplace(Path(value));
 		});
 	}