The backdrop behind a modal dialog took clicks with Sense::click(), which
egui makes focusable, so Tab stopped on it. It now senses clicks without
taking the keyboard focus.
The Tab tests now check that Tab reaches a known control in each region
instead of only that some widget took the focus: the title bar's three
caption buttons, a control inside each dialog, a Tools panel button and a
floating strip's Put back in the toolbar button, and the isolation bar's
back button. The dialog test also checks that Tab skips the backdrop.