Fix early tab change handlers being replaced

Author: NichtStudioCode

invui/src/main/java/xyz/xenondevs/invui/gui/TabGuiImpl.java

@@ -240,7 +240,7 @@ public TabGui build() {
                 throw new IllegalStateException("Structure is not defined.");
 
             var gui = new TabGuiImpl(structure, tab, tabs);
-            gui.setTabChangeHandlers(tabChangeHandlers);
+            tabChangeHandlers.forEach(gui::addTabChangeHandler);
             applyModifiers(gui);
 
             return gui;