sndio: Add module.

- can control sndio: change volume, toggle mute
- appearance is somewhat dynamic: takes muted status into account
- uses polling inside sleeper thread to update values
- uses sioctl_* functions, requires sndio>=1.7.0.

include/modules/sndio.hpp

@@ -0,0 +1,29 @@
+#pragma once
+
+#include <sndio.h>
+#include <vector>
+#include "ALabel.hpp"
+#include "util/sleeper_thread.hpp"
+
+namespace waybar::modules {
+
+class Sndio : public ALabel {
+ public:
+  Sndio(const std::string&, const Json::Value&);
+  ~Sndio();
+  auto update() -> void;
+  auto set_desc(struct sioctl_desc *, unsigned int) -> void;
+  auto put_val(unsigned int, unsigned int) -> void;
+  bool handleScroll(GdkEventScroll *);
+  bool handleToggle(GdkEventButton* const&);
+
+ private:
+  util::SleeperThread thread_;
+  struct sioctl_hdl *hdl_;
+  std::vector<struct pollfd> pfds_;
+  unsigned int addr_;
+  unsigned int volume_, old_volume_, maxval_;
+  bool muted_;
+};
+
+}  // namespace waybar::modules