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/pipewire/pipewire_backend.hpp

@@ -29,7 +29,7 @@ class PipewireBackend {
  public:
   sigc::signal<void> privacy_nodes_changed_signal_event;
 
-  std::unordered_map<uint32_t, PrivacyNodeInfo*> privacy_nodes;
+  std::unordered_map<uint32_t, PWPrivacyNodeInfo*> privacy_nodes;
   std::mutex mutex_;
 
   static std::shared_ptr<PipewireBackend> getInstance();