feat(qmetaobject): add "is" wrappers for special QJSValues

[gikari]

I hope, I handled enum right.

Ref #221

[ratijas]

LGTM, except unrelated qml_register_module formatting change.

[gikari]

It appears, that Default is already used by cpp_class! macros. I do not know if it's fixable.

[ratijas]

It appears, that Default is already used by cpp_class! macros. I do not know if it's fixable.

From the link you referenced:

and Default if the class is default constructible

QJSValue does not explicitly define a standalone default constructor, but it does have one with a single optional argument:

QJSValue::QJSValue(QJSValue::SpecialValue value = UndefinedValue)

so a QJSValue::default() constructor should already be the same as calling undefined(). Worth a test though :)

[gikari]

so a QJSValue::default() constructor should already be the same as calling undefined(). Worth a test though :)

Added as an assert to is_undefined test.

[gikari]

Ping?