Make use of WFixedPanel for the config gui

src/main/kotlin/moe/nea/firmament/gui/WFixedPanel.kt

@@ -0,0 +1,25 @@
+package moe.nea.firmament.gui
+
+import io.github.cottonmc.cotton.gui.widget.WPanel
+import io.github.cottonmc.cotton.gui.widget.WWidget
+
+class WFixedPanel() : WPanel() {
+    var child: WWidget
+        set(value) {
+            children.clear()
+            setSize(0, 0)
+            children.add(value)
+        }
+        get() = children.single()
+
+    constructor(child: WWidget) : this() {
+        this.child = child
+    }
+
+    override fun layout() {
+        setSize(0, 0)
+        super.layout()
+    }
+
+    override fun canResize(): Boolean = false
+}