Tables: focused headers scroll into view; long tables scroll sideways

A sortable header is a tracked Tab stop; once focused it sets its
table's sideways offset whole from its painted bounds and asks one
frame. The virtualized branch scrolls sideways too, and its list
keeps a plain wheel to its rows. layout::on_axis keeps a scroll box's
wheel to its axes; tables, code blocks, the thumbnail strip and the
gallery's page use it. Footer figures move to tables/foot.rs; the
profiler's ruler takes theme.chart().label. HumanInputRequest's
answered state gets a test.

src/debug/timeline.rs

@@ -136,7 +136,7 @@ impl RenderOnce for TimelineProfiler {
         let (from, to) = self.range;
         let length = to - from;
         let at = move |time: f64| ((time - from) / length) as f32;
-        let room = (theme.label_width() * 0.4).to_pixels(window.rem_size());
+        let room = theme.chart().label.to_pixels(window.rem_size());
         let width = lane.read(cx).size.width;
         let (step, ticks) = ruler(self.range, f32::from(width), f32::from(room));
         let wheel_lane = lane.clone();