Add `gst::Allocator` subclassing support
This MR aims to resolve #412 (closed).
I'm not familiar with GLib so please review carefully!
I added very basic unit tests that pass. However test_allocator_alloc
emits this for some reason:
(gstreamer-9dd60b999ffadbaa:68221): GStreamer-CRITICAL **: 12:18:52.807: gst_mini_object_unref: assertion 'GST_MINI_OBJECT_REFCOUNT_VALUE (mini_object) > 0' failed
If you have test cases in mind, let me know and I'll add them.