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/layout/mod.rs

@@ -21,7 +21,7 @@ pub use floating::FloatingPanel;
 pub use grid::{Masonry, SimpleGrid};
 pub use page::{AppShell, MasterDetail, Page};
 pub use panes::{PaneGroup, PaneId, PaneLayout};
-pub use scroll::{ScrollArea, Scrollbar};
+pub use scroll::{ScrollArea, Scrollbar, on_axis};
 pub use scroll_aids::{ScrollShadow, ScrollToTop, StickyHeader};
 pub use sheet::{Drawer, Sheet};
 pub use sidebar::Sidebar;