The old convention was easy to confuse with ScrArea.
Part of https://developer.blender.org/T74432.
This is mostly a batch rename with some manual fixing. Only single word
variable names are changed, no prefixed/suffixed names.
Brecht van Lommel and Campbell Barton both gave me a green light for
this convention change.
Also ran clan 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