The scrollable tab bar previously worked only for left/right
(vertical) tab bars. Generalize the scroll machinery to be
axis-neutral so top/bottom (horizontal) bars scroll too.

- Axis-neutral scroll offset, leading margin, viewport length,
  content extent, and clamping; scrollWheel handles a trackpad
  horizontal swipe and shift+vertical wheel for horizontal bars.
- New horizontal natural-width layout branch that scrolls instead
  of overflowing. Honors the "uneven tab widths allowed" setting:
  equal-width tabs use a uniform width, uneven tabs use natural
  widths with a floor. A new advanced setting, "Tab width when the
  tab bar is scrollable" (default 120), controls that width,
  distinct from the non-scrollable optimum width.
- drawRect clips cells to the scrollable region and repaints the
  leading decoration band and trailing add-button gap so scrolled
  cells neither show under the window decorations nor draw over the
  button. Minimal keeps its selected-tab notch and bar-edge line
  correct at the button edge, including a cap border when the
  active tab straddles it.
- Fix a Yosemite margin-fill that painted nothing for an empty
  clip rect, and extend Minimal's start/end insets across the
  scroll margins so the decoration color fills them (no bare or
  double-drawn strips on a transparent window).
- Drag assistant lays cells out at their scrolled positions.