GstElement: Add a more flexible way to get request pads.

GstElement: Add a more flexible way to get request pads.

The new request_new_pad_full vmethod provides an additional caps field,
which allows elements to take better decision process.

Also, add a gst_element_request_pad() function to allow developers to be
able to specify which pad template they want a pad of.

Convert gstutils to use that new method instead of the old one when more
efficient.

This is useful for being able to request pads in a more flexible way,
especially when the element can provide pads whose caps depend on
runtime configuration and therefore can't provide pre-registered
pad templates.

API: GstElement::request_new_pad_full
API: gst_element_request_pad

https://bugzilla.gnome.org/show_bug.cgi?id=637300