pad: add GST_PAD_FLAG_ACCEPT_TEMPLATE

pad: add GST_PAD_FLAG_ACCEPT_TEMPLATE

It will make the default accept-caps handler use the pad template
caps instead of the query-caps result to check if the caps is
acceptable. This is aligned with what the design docs says the
accept-caps should do (be non-recursive) and should be faster. It
is *not* enabled by default, though.

API: GST_PAD_FLAG_ACCEPT_TEMPLATE
API: GST_PAD_IS_ACCEPT_TEMPLATE
API: GST_PAD_SET_ACCEPT_TEMPLATE
API: GST_PAD_UNSET_ACCEPT_TEMPLATE

https://bugzilla.gnome.org/show_bug.cgi?id=753623