gst/: Make it possible (and recommended) to set element details and add pad templates in the class_init functions by ...

Original commit message from CVS:
* gst/gstelement.c: (gst_element_base_class_init),
(gst_element_class_add_pad_template):
* gst/gstpadtemplate.c:
Make it possible (and recommended) to set element details and add
pad templates in the class_init functions by copying the details/pad
templates in GstElement's base_init.
Also make it possible to replace existing pad templates by adding
a new one with the same name. This was done in a hackish fashion
in same elements before already.
Don't reference pad templates that are added a second time. A
new pad template has a refcount of one and is not floating anymore
and to be owned by the element's class. Make this more explicit by
mentioning it in the docs of gst_element_class_add_pad_template().
These changes are backwards compatible. Fixes bug #491501.
* tests/check/gst/gstelement.c:
Add unit test for setting element details, adding pad templates and
replacing them in a subclass.