Change func got collected before triggering

[kezhuw]

There is no strong reference to callback in java for GstPromiseAPI.ptr_gst_promise_new_with_change_func in Promise's constructor.

[neilcsmith-net]

Yes, that looks rather broken! 🙄 Not sure of best way to fix that - perhaps needs private constructor taking a GstCallback?

[neilcsmith-net]

Should be fixed by #200 - please take a look and comment if need be - will aim to merge and release with 1.2 next week.