T21a round 2 fixes

- a field notes its selection when a key goes down, so undo of typing selects
  what the typing replaced
- reading order skips table cells merged away, so Up and Down across blocks
  land on cells that show

src/documents/blocks/view.rs

@@ -114,6 +114,7 @@ impl BlockEditor {
                 cx.stop_propagation();
                 editor.on_submit(key, window, cx);
             }))
+            .capture_key_down(cx.listener(move |editor, _, _, cx| editor.note_key(key, ix, cx)))
             .on_mouse_down(MouseButton::Left, |_, _, cx| cx.stop_propagation())
             .child(field.clone());
         let slash = move |query: &str, at: usize, caret: usize| -> (Vec<Choice>, Pick) {