Refactor the `recalcData_nla()` function, which takes data from the
transform system and updates NLA strips, such that the actual logic to
change the strip is moved into its own function.
This also moves some generic code (find prev/next strip) from that
function to BKE.
This is to make the code easier to adjust when different transform
operations need to perform different modifications of the strip (i.e. to
fix T101130).
Manifest Task: T101130
Reviewed By: sybren
Differential Revision: https://developer.blender.org/D16181
962b647690