Add backlight module

Monitor the backlight level via udev. Poll every `interval` as well,
in case backlight udev events aren't supported.

include/modules/backlight.hpp

@@ -0,0 +1,65 @@
+#pragma once
+
+#include <optional>
+#include <string>
+#include <string_view>
+#include <vector>
+
+#include "ALabel.hpp"
+#include "util/json.hpp"
+#include "util/sleeper_thread.hpp"
+
+struct udev;
+struct udev_device;
+
+namespace waybar::modules {
+
+class Backlight : public ALabel {
+  class BacklightDev {
+  public:
+    BacklightDev() = default;
+    BacklightDev(std::string name, int actual, int max);
+    std::string_view name() const;
+    int get_actual() const;
+    void set_actual(int actual);
+    int get_max() const;
+    void set_max(int max);
+    friend inline bool operator==(const BacklightDev &lhs,
+                                  const BacklightDev &rhs) {
+      return lhs.name_ == rhs.name_ && lhs.actual_ == rhs.actual_ &&
+             lhs.max_ == rhs.max_;
+    }
+
+  private:
+    std::string name_;
+    int actual_ = 1;
+    int max_ = 1;
+  };
+
+public:
+  Backlight(const std::string &, const Json::Value &);
+  ~Backlight();
+  auto update() -> void;
+
+private:
+  template <class ForwardIt>
+  static const BacklightDev *best_device(ForwardIt first, ForwardIt last,
+                                         std::string_view);
+  template <class ForwardIt, class Inserter>
+  static void upsert_device(ForwardIt first, ForwardIt last, Inserter inserter,
+                            udev_device *dev);
+  template <class ForwardIt, class Inserter>
+  static void enumerate_devices(ForwardIt first, ForwardIt last,
+                                Inserter inserter, udev *udev);
+
+  const std::string name_;
+  const std::string preferred_device_;
+  static constexpr int EPOLL_MAX_EVENTS = 16;
+
+  waybar::util::SleeperThread udev_thread_;
+  std::mutex udev_thread_mutex_;
+  std::vector<BacklightDev> devices_;
+
+  std::optional<BacklightDev> previous_best_;
+};
+} // namespace waybar::modules