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.

docs/guest.md

@@ -48,10 +48,11 @@ place in it waits for the host to return.
 
 ## The screen
 
-Across the top is the toolbar, one row holding everything: the tools
-and their settings, the view, the host's page buttons, your notebook,
-and **Session**, which opens a window with the participants and the
-invitation.  Below it the page has the whole width of the window.  Every button
+Across the top is the toolbar, one row holding everything in three
+groups: on the left the tools, their settings and **Undo**; in the
+middle the page and the view; on the right the host's page buttons,
+your notebook, and **Session**, which opens a window with the
+participants and the invitation.  Below it the page has the whole width of the window.  Every button
 explains itself: rest the pointer on it for two seconds and its
 explanation appears beneath it.
 
@@ -117,7 +118,24 @@ and to the next ones.
 **Eraser** removes your own writing.  Move it across a stroke and the
 whole stroke goes; across a text block and the block goes.  It removes
 nothing of anyone else's, and nobody else's eraser can touch yours.  An
-erased stroke is gone from the page for everyone.
+erased stroke is gone from the page for everyone, until you undo the
+erasure.
+
+### Undo
+
+**Undo**, the last button of the drawing tools, takes back your latest mark: a
+stroke, what one movement of the eraser removed, a text block as you
+last typed or moved it, a Hide rectangle made, moved or removed.  Press
+it again for the mark before that, and so on back to the beginning of
+the page.  Ctrl+Z (Command+Z on a Mac) does the same, and so do two
+quick taps of the Eraser on empty paper, away from anything the eraser
+would remove; a tap that erases something is never half of such a
+double tap.  Undo acts only on your own marks, since nobody can change
+anyone else's, and only on marks made on this page since you opened it:
+after a reload of the tab, or on a page loaded from a notebook, there is
+nothing to undo although the marks are there.  The button is grey when
+there is nothing to undo.  An image cannot be undone; put it back with
+**Image** if a tap of the eraser removed it by mistake.
 
 ### The view