feat(render): support pseudo-transparency

This adds pseudo-transparency for the background of the bar and the
background of the systray.

src/components/controller.cpp

@@ -686,4 +686,10 @@ bool controller::on(const signals::ipc::hook& evt) {
   return true;
 }
 
+bool controller::on(const signals::ui::update_background&) {
+  enqueue(make_update_evt(true));
+
+  return false;
+}
+
 POLYBAR_NS_END