Import keyframes dialog: add two new modes offset and inverted position

Fixes #438

The two new modes are useful to paste the analysis date from the
opencv.tracker (aka Motion Tracker) effect to a transform effect and use
them to stabilize the footage.

Inverted Position is the same than Position but with negativ values x >
-x and y > -y

Offset position is the difference beetween the reference point's position
(first keyframe) and the current keyframe's position