First stage of DrawText upgrade.
- Rearrange code into three distinct areas; DrawText, GrayString,
  TabbedText.
- Remove unused macros.
- Modify treatment of Tab arguments to fit with observed behaviour.
  e.g. DrawTextEx with DT_TABSTOP but null dtp will not take the
  setting from the flags.  Also only uses bits 15 to 8, not the
  higher bits.