Undo, and the toolbar in three groups

Undo takes back one's own latest mark, by the button, Ctrl+Z or two
quick taps of the eraser; the toolbar's drawing tools, the page and
view, and the session stand in three groups that do not move when the
tool changes; the buttons are a little smaller.

client/js/selection_guard.js

@@ -46,7 +46,7 @@ function describe(node) {
 }
 
 /** Whether the selection belongs to a focused text field. */
-function inField() {
+export function inField() {
   const active = document.activeElement;
   return active instanceof HTMLInputElement || active instanceof HTMLTextAreaElement ||
          (active instanceof HTMLElement && active.isContentEditable);