v0.1.0-preview.4

Added

• Xtreme.Components.Cards: SectionCard.HeaderFit. Set it to SectionCardHeaderFit.Abbreviate
  and the tab headers shorten as the card narrows, down to icon only, before any tab moves into
  the overflow menu. The fit is measured (each header at the strip's real font against the strip's
  real width), not a breakpoint table, and a shortened tab shows its full header as a tooltip. The
  default, Overflow, keeps today's behavior.
• Xtreme.Components.Cards: LabelAbbreviation, the shortening rule behind it (first N letters
  plus ".."; zero letters means icon only), public so hosts can shorten their own labels the same way.
• Gallery: the Cards page has a header-fitting demo, a width slider over an Abbreviate card and
  an Overflow card.

Changed

• Xtreme.Components.Cards: SectionCard's tab strip no longer reserves NavigationView's empty
  48px PaneCustomContent column, so more tabs fit before the overflow menu appears.