Support marshaling generic Godot.Object

Allows using generic C# types in signals as long as they inherit
from `Godot.Object`.

modules/mono/mono_gd/gd_mono_field.cpp

@@ -493,6 +493,13 @@ void GDMonoField::set_value_from_variant(MonoObject *p_object, const Variant &p_
 				mono_field_set_value(p_object, mono_field, managed);
 				break;
 			}
+
+			// GodotObject
+			GDMonoClass *type_class = type.type_class;
+			if (CACHED_CLASS(GodotObject)->is_assignable_from(type_class)) {
+				MonoObject *managed = GDMonoUtils::unmanaged_get_managed(p_value.operator Object *());
+				mono_field_set_value(p_object, mono_field, managed);
+			}
 		} break;
 
 		default: {

modules/mono/mono_gd/gd_mono_marshal.cpp

@@ -206,6 +206,12 @@ Variant::Type managed_to_variant_type(const ManagedType &p_type) {
 			if (GDMonoUtils::Marshal::type_is_generic_icollection(reftype) || GDMonoUtils::Marshal::type_is_generic_ienumerable(reftype)) {
 				return Variant::ARRAY;
 			}
+
+			// GodotObject
+			GDMonoClass *type_class = p_type.type_class;
+			if (CACHED_CLASS(GodotObject)->is_assignable_from(type_class)) {
+				return Variant::OBJECT;
+			}
 		} break;
 
 		default: {
@@ -682,6 +688,12 @@ MonoObject *variant_to_mono_object(const Variant *p_var, const ManagedType &p_ty
 
 					return GDMonoUtils::create_managed_from(p_var->operator Array(), godot_array_class);
 				}
+
+				// GodotObject
+				GDMonoClass *type_class = p_type.type_class;
+				if (CACHED_CLASS(GodotObject)->is_assignable_from(type_class)) {
+					return GDMonoUtils::unmanaged_get_managed(p_var->operator Object *());
+				}
 			} break;
 		} break;
 	}
@@ -878,6 +890,17 @@ Variant mono_object_to_variant_impl(MonoObject *p_obj, const ManagedType &p_type
 				GDMonoUtils::Marshal::array_get_element_type(reftype, &elem_reftype);
 				return system_generic_list_to_Array_variant(p_obj, p_type.type_class, elem_reftype);
 			}
+
+			// GodotObject
+			GDMonoClass *type_class = p_type.type_class;
+			if (CACHED_CLASS(GodotObject)->is_assignable_from(type_class)) {
+				Object *ptr = unbox<Object *>(CACHED_FIELD(GodotObject, ptr)->get_value(p_obj));
+				if (ptr != NULL) {
+					Reference *ref = Object::cast_to<Reference>(ptr);
+					return ref ? Variant(Ref<Reference>(ref)) : Variant(ptr);
+				}
+				return Variant();
+			}
 		} break;
 	}