timedvaluecontrolsource: Add some signals about values changes

timedvaluecontrolsource: Add some signals about values changes

In order for user to be able to track changes in the value set in
GstTimedValueControlSource the following signals have been added:
 * value-added
 * value-removed
 * value-changed

To be able to use a GstControlPoint to be marshalled into the signals,
the GstControlPoint structure is now registerd as a GBoxed type.

New API:
~~~~~~~
  * GstTimedValueControlSource::value-added
  * GstTimedValueControlSource::value-removed
  * GstTimedValueControlSource::value-added

https://bugzilla.gnome.org/show_bug.cgi?id=737616