unmark changed callback as experimental

Closes #112

docs/reference/src/language/syntax/properties.md

@@ -151,11 +151,8 @@ export component Example inherits Window {
 }
 ```
 
-<!--
 ## Change Callbacks
 
-**This feature is still experimental**
-
 In Slint, it's possible to define a callback that is invoked when a property's value changes.
 
 ```slint
@@ -213,5 +210,3 @@ Using a changed callback forces immediate evaluation of bindings, which are typi
 This practice also compromises the purity of bindings, complicating edits via graphical editors.
 Accumulating excessive changed events can introduce issues and bugs, especially in scenarios involving loops, where a change callback modifies a property, potentially triggering changes to the same property.
 
-
--->

examples/weather-demo/build.rs

@@ -1,10 +1,6 @@
 // Copyright © SixtyFPS GmbH <info@slint.dev>
 // SPDX-License-Identifier: MIT
 
-use std::env;
-
 fn main() {
-    env::set_var("SLINT_ENABLE_EXPERIMENTAL_FEATURES", "true");
-
     slint_build::compile("ui/main.slint").unwrap();
 }

internal/compiler/object_tree.rs

@@ -1354,12 +1354,6 @@ impl Element {
         }
 
         for ch in node.PropertyChangedCallback() {
-            if !diag.enable_experimental && !tr.expose_internal_types {
-                diag.push_error(
-                    "Change callbacks are experimental and not yet implemented in this version of Slint".into(),
-                    &ch,
-                );
-            }
             let Some(prop) = parser::identifier_text(&ch.DeclaredIdentifier()) else { continue };
             let lookup_result = r.lookup_property(&prop);
             if !lookup_result.is_valid() {