GMLCompiler: Add common pattern for a user-defined fallible initializer

Many widget classes need to run substantial initialization code after
they have been setup from GML. With this change, an
initialize_fallibles() function is called if available, allowing the
initialization to be invoked from the GML setup automatically. This
means that the GML-generated creation function can now be used directly
for many more cases, and reduces code duplication.

Base/usr/share/man/man5/GML/Usage.md

@@ -50,3 +50,16 @@ From there, you can use `find_descendant_of_type_named` to select widgets from y
 // MyApp::Widget::foo_bar
 m_mem_add_button = *find_descendant_of_type_named<GUI::Button>("mem_add_button");
 ```
+
+### `initialize` Pattern
+
+Initialization, like adding models, attaching callbacks, etc., should be done in a member function with the signature:
+
+```cpp
+// MyApp::Widget
+ErrorOr<void> initialize();
+```
+
+This initializer function, if it exists, will automatically be called after the structure of your widget was set up by the auto-generated GML code.
+
+The only case where this function cannot be used is when your initialization requires additional parameters, like a GUI window. You may still consider moving as much initialization to the canonical function as possible.

Meta/Lagom/Tools/CodeGenerators/GMLCompiler/main.cpp

@@ -334,6 +334,8 @@ static ErrorOr<void> generate_loader_for_object(GUI::GML::Object const& gml_obje
         return {};
     }));
 
+    TRY(append(generator, "TRY(::GUI::initialize(*@object_name@));"));
+
     generator.append(TRY(String::repeated(' ', (indentation - 1) * 4)).bytes_as_string_view());
     generator.appendln("}");
 

Userland/Libraries/LibGUI/Widget.h

@@ -71,6 +71,15 @@ enum class AllowCallback {
     Yes
 };
 
+template<typename T>
+ALWAYS_INLINE ErrorOr<void> initialize(T& object)
+{
+    if constexpr (requires { { object.initialize() } -> SameAs<ErrorOr<void>>; })
+        return object.initialize();
+    else
+        return {};
+}
+
 class Widget : public GUI::Object {
     C_OBJECT(Widget)
 public: