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/tables/mod.rs

@@ -1,6 +1,7 @@
 mod body;
 mod builders;
 mod cell;
+mod foot;
 mod formula;
 mod grid;
 mod gridkeys;