Doing it in gst_play_new() means that bindings that directly call
g_object_new() with the GType wouldn't end up initializing both.
This affects at least the Python and GJS bindings.
gst_init() is nonetheless only called from gst_play_new() once because
calling it from class_init would likely lead to problems as that's
called from somewhere in the middle of GObject.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6821>
