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"
  }

src/modules/pulseaudio_slider.cpp

@@ -6,6 +6,7 @@ PulseaudioSlider::PulseaudioSlider(const std::string& id, const Json::Value& con
     : ASlider(config, "pulseaudio-slider", id) {
   backend = util::AudioBackend::getInstance([this] { this->dp.emit(); });
   backend->setIgnoredSinks(config_["ignored-sinks"]);
+  backend->setSinkMapping(config_["sink-mapping"]);
 
   if (config_["target"].isString()) {
     std::string target = config_["target"].asString();
@@ -79,4 +80,4 @@ void PulseaudioSlider::onValueChanged() {
   backend->changeVolume(volume, min_, max_);
 }
 
-}  // namespace waybar::modules
+}  // namespace waybar::modules