debug: Instantiate GType when dumping debug categories.

debug: Instantiate GType when dumping debug categories.

A lot of debug categories are declared in element class_init
functions, which don't get run until the element is first created
(not just registered in the plugin load function). This means
that --gst-debug-help doesn't print out a lot of categories.

Creating an instance of each element from the element factory
makes them visible, at some extra cost - 2-3 times longer, which can
be a full second or two of extra waiting. Yikes!

https://bugzilla.gnome.org/show_bug.cgi?id=741001