Remove gst_mini_object_register() and add a GST_DEFINE_MINI_OBJECT macro to
define a _get_type() function for the boxed miniobject.
Remove a bunch of custom _get_type() functions and replace them with the
miniobject macro.
Rename some _init method to _priv_*_initialize() like the rest of them.

Inspired by patch from Johan Dahlin and see bug #657603