Animation: access all possibly animated values through getter functions

in some cases we need to retrieve those values for a different frame than
the global "current frame". Seemingly, Blender doesn't provide infrastructure
for such a situation -- thus we need to build our own infrastructure.
Animation: allocate private working data (API CHANGE)

maintain a private struct for caching the F-curves.

Warning: the signature of BKE_tracking_data_get() has changed.
We need the full clip data, to be able to remap the frame number
to scene coordinates.
Animation: retrieve values from F-curves

On initialisation, do a lookup of existing animation F-curves.
This allows us to use the correct parameter values, especially
for initialisation of the per track reference frame and baseline values.