Accessibility: User-configurable selection hitbox accuracy

Selection in blender uses a hard-coded hitbox size, meaning the user has no control over click accuracy. For accessibility purposes the hitbox size should be user-configurable in Blender's input preferences.

While I think this is important for mouse users, it will likely also become important for Android/iOS as Blender dives more fully into native tablet builds to allow for variability in tablet input precision.

I'm not positive but I believe the size of the hitbox is controlled by

float ED_view3d_select_dist_px()
{
  return 75.0f * U.pixelsize;
}

in view3d_select.cc