Refactor source layout

Introduce compat source sets and move all kotlin sources to the main directory

[no changelog]

src/main/kotlin/util/customgui/HasCustomGui.kt

@@ -0,0 +1,17 @@
+
+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)
+    }
+