Draft: Add call and is_callable to QJSValue

[gikari]

Summary

This adds the call and is_callable wrappers for QJSValue. Unfortunately, callables cannot be tested without QJSEngine or equivalent.

Dependencies

Depends on #224

Help needed

I currently cannot solve these problems:

1. There can be only one QmlEngine, so if you execute all the tests, the new one panics. I see, that the tests, that are using QmlEngine are using the mutex, but they are in completely different module, so I have no idea how to use the same mutex, as them.
2. I really don't like the duplication of the QJSValueList, QVariantList and QStringList. Is there a way to avoid this kind of stuff, maybe using generics and macro?

Related issues

#221