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.

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: