io/ipchandler: add IpcHandler and qs msg

Also reworks the whole ipc system to use serialized variants.

src/core/ipccommand.hpp

@@ -0,0 +1,20 @@
+#pragma once
+
+#include <variant>
+
+#include "../io/ipccomm.hpp"
+#include "ipc.hpp"
+
+namespace qs::ipc {
+
+struct IpcKillCommand: std::monostate {
+	static void exec(IpcServerConnection* /*unused*/);
+};
+
+using IpcCommand = std::variant<
+    std::monostate,
+    IpcKillCommand,
+    qs::io::ipc::comm::QueryMetadataCommand,
+    qs::io::ipc::comm::StringCallCommand>;
+
+} // namespace qs::ipc