pulseaudio: Add 'sink-mapping' config option

This commit adds a new optional config to the pulseaudio modules. It
maps sink names to other sink names so that if the current sink is a
key, the sink named by the value is considered to be the current sink
instead of it.

E.g.
  "sink-mapping": {
    "easyeffects_sink": "speakers_sink"
  }

include/util/audio_backend.hpp

@@ -48,6 +48,7 @@ class AudioBackend {
   std::string default_source_name_;
 
   std::vector<std::string> ignored_sinks_;
+  std::map<std::string, std::string> sink_mapping_;
 
   std::function<void()> on_updated_cb_ = NOOP;
 
@@ -69,6 +70,7 @@ class AudioBackend {
   void changeVolume(ChangeType change_type, double step = 1, uint16_t max_volume = 100);
 
   void setIgnoredSinks(const Json::Value& config);
+  void setSinkMapping(const Json::Value& config);
 
   std::string getSinkPortName() const { return port_name_; }
   std::string getFormFactor() const { return form_factor_; }
@@ -93,4 +95,4 @@ class AudioBackend {
   bool isBluetooth();
 };
 
-}  // namespace waybar::util
+}  // namespace waybar::util