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/util/audio_backend.cpp

@@ -155,6 +155,13 @@ void AudioBackend::sinkInfoCb(pa_context * /*context*/, const pa_sink_info *i, i
     }
   }
 
+  if (const auto mapping = backend->sink_mapping_.find(backend->current_sink_name_);
+      mapping != backend->sink_mapping_.end()) {
+    if (i->name == mapping->second) {
+      backend->current_sink_name_ = i->name;
+    }
+  }
+
   if (backend->current_sink_name_ == i->name) {
     backend->current_sink_running_ = i->state == PA_SINK_RUNNING;
   }
@@ -292,4 +299,14 @@ void AudioBackend::setIgnoredSinks(const Json::Value &config) {
   }
 }
 
+void AudioBackend::setSinkMapping(const Json::Value &config) {
+  if (config.isObject()) {
+    for (auto it = config.begin(); it != config.end(); ++it) {
+      if (it.key().isString() && it->isString()) {
+        sink_mapping_.emplace(it.key().asString(), it->asString());
+      }
+    }
+  }
+}
+
 }  // namespace waybar::util