Add new builtin keying sets for sequencer to allow for quickly
animating image location, rotation and scale. This is useful feature,
that improves workflow when animating image transformation.
It is possible to insert keyframes to custom keying sets in context of
preview region, but since these properties can be changed only from
side panel, it does not make sense to add these to builtin sets at this
moment.
Poll functions has been refactored to allow other editors to be used.
RKS_POLL_selected_items was replaced by
RKS_POLL_selected_objects_bones.
Refactoring was also necessary for "Available" keyng sets, since it was
assumed, that object is being animated. To work within sequencer and
3D view context, functions RKS_POLL_view_3d and
RKS_POLL_selected_sequences were created.
Example:

