Button UI that can paint rotated text, which is used by
BasicSlidingTabDisplayerUI .
Uses the lightweight HTML renderer for good performance when rendering HTML strings. It is
intended as a UI for
BasicSlidingTabDisplayerUI.IndexButton . Provide a subclass
of this class via UIDefaults to change the painting behavior or appearance of the tab buttons in
"sliding" style tabbed controls. Typically the only method of interest when
subclassing is
SlidingTabDisplayerButtonUI.paintBackground .