overrides: add a set_caps() method to the Pad override

The C API provides the gst_pad_set_caps() helper which makes it easier
to set caps on pads (see gst/gstcompat.h in gstreamer core).

Add such handy helper to the python bindings too.

The implementation follows as close as possible the one in gstcompat.h
with two changes:
  1. the type check on the pad has been removed because self is
     guaranteed to be a Gst.Pad in python.
  2. the null check on the caps has been extended to be a type check.

Fixes #19
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