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.
- Add 'Navigation Buttons' preference, used for 2D views
(previously this couldn't be disabled).
- Add "Off" option for 3D view axis.
- Support minimal axis with navigation buttons.