Follow up of b2ee1770d4c3 and 10c2254d412d, part of T74432.
Now the area and region naming conventions should be less confusing.
Mostly a careful batch rename but had to do few smaller fixes.
Also ran clang-format on affected files.
Part of T68836
`transform conversions.c` is a file that is getting too big (almost 10,000 lines).
So it's a good idea to split it into smaller files.
differential revision: https://developer.blender.org/D5677
