Added GeoClue2 privacy item

Rebased onto current master and fixed the privacy module build:
- privacy_item.hpp no longer includes privacy.hpp (it does not use the
  Privacy class); this broke the circular include that left PrivacyItem
  undeclared when privacy.hpp was reached first from privacy_item.cpp.
- privacy_item.cpp now includes gtkmm/label.h explicitly, since Gtk::Label
  was previously only pulled in transitively via privacy.hpp.

include/util/geoclue_backend.hpp

@@ -0,0 +1,43 @@
+#pragma once
+
+#include <giomm/dbusconnection.h>
+
+#include <atomic>
+
+#include "giomm/dbusproxy.h"
+
+namespace waybar::util::GeoClueBackend {
+
+class GeoClueBackend {
+ private:
+  guint watcherID_;
+  sigc::connection signal_conn;
+  Glib::RefPtr<Gio::DBus::Proxy> proxy;
+  bool connected;
+
+  /* Hack to keep constructor inaccessible but still public.
+   * This is required to be able to use std::make_shared.
+   * It is important to keep this class only accessible via a reference-counted
+   * pointer because the destructor will manually free memory, and this could be
+   * a problem with C++20's copy and move semantics.
+   */
+  struct PrivateConstructorTag {};
+
+ public:
+  sigc::signal<void> in_use_changed_signal_event;
+
+  std::atomic<bool> location_in_use;  // GeoClue is being used
+
+  static std::shared_ptr<GeoClueBackend> getInstance();
+
+  // DBus callbacks
+  void onAppear(const Glib::RefPtr<Gio::DBus::Connection>&, const Glib::ustring&,
+                const Glib::ustring&);
+  void onVanished(const Glib::RefPtr<Gio::DBus::Connection>&, const Glib::ustring&);
+  void propertyChanged(const Gio::DBus::Proxy::MapChangedProperties& changedProperties,
+                       const std::vector<Glib::ustring>& invalidatedProperties);
+
+  GeoClueBackend(PrivateConstructorTag tag);
+  ~GeoClueBackend();
+};
+}  // namespace waybar::util::GeoClueBackend