play: Initialize debug category and error quark in class_init

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.