This avoids need to do special trickery detecting whether the principal
point is to be changed when reloading movie clip. This also allows to
transfer the optical center from high-res footage to possibly its lower
resolution proxy without manual adjustment.

On a user level the difference is that the principal point is exposed in
the normalized coordinates: frame center has coordinate of (0, 0), left
bottom corner of a frame has coordinate of (-1, -1) and the right top
corner has coordinate of (1, 1).

Another user-visible change is that there is no more operator for setting
the principal point to center: use backspace on the center sliders will
reset values to 0 which corresponds to the center.

The code implements versioning in both directions, so it should be
possible to open file in older Blender versions without loosing
configuration.

From the python API a new property is to be accessed: it is now called
principal_point. This is more descriptive name, and allows to clearly
catch cases when the old-style pixel value is assigned to to a property
which expects to be in the normalized space.