Fix concurrent execution of JS API calls (#400)

* [All] Fix JS API to support multiple function invocations

* Fix tests

* [GTK] Fix JS API call

* Update dlls

* Add test for concurrent js_api calls

* Revert js_api example

* Update appveyor conf

* Fix appveyor.yml

* Fix appveyor conf

* Fix appveyor conf

* Fix appveyor conf

* Fix appveyor conf

* Fix appveyor conf

* Fix appveyor conf

* Update appveyor conf

* [Windows] Disable concurrent test