element: add gst_element_class_add_static_pad_template()

element: add gst_element_class_add_static_pad_template()

Pretty much every single element does

  gst_element_class_add_pad_template (element_class,
      gst_static_pad_template_get (&some_templ));

which is both confusing and unnecessary. We might just
as well add a function to do that in one step.

https://bugzilla.gnome.org/show_bug.cgi?id=762778