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 (closed)