Added because the current default is too fast
for painting with tablets, see D5385.
Turntable and trackball have different settings because
turn-table uses an angle-per-pixel, where as trackball
values are relative to the view-port size so a scale is used.
The sensitivity is scaled by the pixel size so hi-dpi views don't rotate faster.
Simplify preferences by removing the ability to load them from
either the startup.blend or userpref.blend.
Also simplifies updating default preferences by moving
them to a struct definition.
