gstpad.override: Take a copy of gst_static_pad_template_get_caps()

gstpad.override: Take a copy of gst_static_pad_template_get_caps()

This means that we take a completely new caps for the sole usage of
gst-python. The GstCaps return by gst_static_pad_template_get_caps() are
(surprise) static and therefore will always exist... as long as the
GstStaticPadTemplate (and the factory providing it) still exist.

This solves the case of getting the caps of a static pad template *before*
any element was created using the GstElementFactory. When the factory is
used to create an element, a new factory is created, replacing the old one,
and plainly discarding any static values (including those caps).