N.B. : this does not modify the functionality of the transform system,
but merely sets the stage for an easier comprehension of the NLA transform
stack application.
This diff adds a function to separate the strip modification from the
recomputation of the NLA data. Since we're going to modify the function
later, it is better to have the NlaStrip-modifying logic to another
function to facilitate code review.
Also, this adds a couple BKE functions for the NlaStrip, in order to find
the previous/next strips in the strip's track.
The next revision will focus on actually fixing T101130.