feat(util): SafeSignal class for cross-thread signals with arguments

Implement a wrapper over Glib::Dispatcher that passes the arguments to
the signal consumer via synchronized `std::queue`.
Arguments are always passed by value and the return type of the signal
is expected to be `void`.

include/util/SafeSignal.hpp

@@ -0,0 +1,59 @@
+#pragma once
+
+#include <glibmm/dispatcher.h>
+#include <sigc++/signal.h>
+
+#include <functional>
+#include <mutex>
+#include <queue>
+#include <tuple>
+#include <type_traits>
+
+namespace waybar {
+
+/**
+ * Thread-safe signal wrapper.
+ * Uses Glib::Dispatcher to pass events to another thread and locked queue to pass the arguments.
+ */
+template <typename... Args>
+struct SafeSignal : sigc::signal<void(std::decay_t<Args>...)> {
+ public:
+  SafeSignal() { dp_.connect(sigc::mem_fun(*this, &SafeSignal::handle_event)); }
+
+  template <typename... EmitArgs>
+  void emit(EmitArgs&&... args) {
+    {
+      std::unique_lock lock(mutex_);
+      queue_.emplace(std::forward<EmitArgs>(args)...);
+    }
+    dp_.emit();
+  }
+
+  template <typename... EmitArgs>
+  inline void operator()(EmitArgs&&... args) {
+    emit(std::forward<EmitArgs>(args)...);
+  }
+
+ protected:
+  using signal_t = sigc::signal<void(std::decay_t<Args>...)>;
+  using arg_tuple_t = std::tuple<std::decay_t<Args>...>;
+  // ensure that unwrapped methods are not accessible
+  using signal_t::emit_reverse;
+  using signal_t::make_slot;
+
+  void handle_event() {
+    auto fn = signal_t::make_slot();
+    for (std::unique_lock lock(mutex_); !queue_.empty(); lock.lock()) {
+      auto args = queue_.front();
+      queue_.pop();
+      lock.unlock();
+      std::apply(fn, args);
+    }
+  }
+
+  Glib::Dispatcher        dp_;
+  std::mutex              mutex_;
+  std::queue<arg_tuple_t> queue_;
+};
+
+}  // namespace waybar