gstframepositioner: added 'operator' property

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.

Related to but does not close Issue
https://gitlab.gnome.org/GNOME/pitivi/-/issues/2313

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-editing-services/-/merge_requests/236>