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.