Fix polygon shape move

Move vector should not affect start/end points as they are assumed to always be 0,0. Polygon is defined by outline only.

Right now move operation affects both start point and the outline of the polygon but only until pcbnew is restarted. Upon reload API returns 0,0 for start point (which makes sense since start/end points are not saved in the file) and only outline is moved.

This leads to inconsistent API results that change between pcbnew restarts which is a bug.