libs/gst/controller/: Factor out the 'set' logic into gst_controller_set_unlocked for the gst_controller_set and gst_...

Original commit message from CVS:
* libs/gst/controller/gstcontroller.c:
(gst_controlled_property_set_interpolation_mode),
(gst_controlled_property_prepend_default),
(gst_controlled_property_new), (gst_controller_set_unlocked),
(gst_controller_set), (gst_controller_set_from_list),
(gst_controller_unset), (gst_controller_unset_all):
* libs/gst/controller/gstcontrollerprivate.h:
* libs/gst/controller/gstinterpolation.c:
Factor out the 'set' logic into gst_controller_set_unlocked for the
gst_controller_set and gst_controller_set_from_list functions.
To make life of the interpolators easier always add a control point
at timestamp zero with the default value.
In the linear interpolator make things more obvious by better variable
naming (slope).
Implement cubic interpolation mode (by using a natural cubic spline)
and map the quadratic interpolation mode to this too (as quadratic
doesn't make much sense, see discussion on the list).
* tests/check/libs/controller.c: (GST_START_TEST),
(gst_controller_suite):
Add unit test for the cubic interpolation mode and check everywhere
if the interpolation mode could be set as expected.