This allows to add pad templates and set metadata in class_init instead of
base_init. base_init is a concept that is not supported by almost all
languages and copying the templates/metadata for subclasses is the more
intuitive way of doing things.

Subclasses can override pad templates of parent classes by adding a new
template with the same now.

Also gst_element_class_add_pad_template() now takes ownership of the
pad template, which was assumed by all code before anyway.

Fixes bug #491501.