make exchange slot available as 'hud.exchange-slot'

src/frontend/hud.cpp

@@ -469,6 +469,7 @@ void Hud::showExchangeSlot() {
         gui,
         SlotLayout(-1, glm::vec2(), false, false, nullptr, nullptr, nullptr)
     );
+    exchangeSlot->setId("hud.exchange-slot");
     exchangeSlot->bind(exchangeSlotInv->getId(), exchangeSlotInv->getSlot(0), &content);
     exchangeSlot->setColor(glm::vec4());
     exchangeSlot->setInteractive(false);