Add interactive storage overlay

src/main/kotlin/moe/nea/firmament/util/customgui/HasCustomGui.kt

@@ -0,0 +1,22 @@
+/*
+ * SPDX-FileCopyrightText: 2024 Linnea Gräf <nea@nea.moe>
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */
+
+package moe.nea.firmament.util.customgui
+
+import net.minecraft.client.gui.screen.ingame.HandledScreen
+
+@Suppress("FunctionName")
+interface HasCustomGui {
+    fun getCustomGui_Firmament(): CustomGui?
+    fun setCustomGui_Firmament(gui: CustomGui?)
+}
+
+var <T : HandledScreen<*>> T.customGui: CustomGui?
+    get() = (this as HasCustomGui).getCustomGui_Firmament()
+    set(value) {
+        (this as HasCustomGui).setCustomGui_Firmament(value)
+    }
+