Minor style cleanup.
DriveSetup: improved size slider.

* It uses size_for_string() to display all sizes which also fixes
  using MiB for one, and MB for the other size string.
* It is no longer limited to 1M granularity by itself, but can have
  arbitrary granularity as long as the number of possible values
  remain smaller than 2^31.
* This means it can also handle values beyond 4TB now.
* The sizes that are set programmatically retain their full resolution.
* CreateParametersPanel still uses 1M granularity for now, though,
  as we would otherwise need to be able to parse a size string with
  a unit.
Minor cleanup.
BSlider::SetPosition() now calls BSlider's SetValue().

* Before it directly called the BControl version which doesn't really
  make any sense, and prevented visual updates.
Minor cleanup.

* Slipped through; forgot to save.