gstframepositioner: added 'operator' property
The 'operator' property was added to gstframepositioner so that blending modes in the compositor could be accessed. This was done by accessing the pad of the compositor class, and referencing the 'operator' property in that pad. Getters and Setters were also created so that the 'operator' could be accessed by software that is based on GES, such as Pitivi.