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/host.md

@@ -195,7 +195,8 @@ anyone included, is covered in white, marked by blue corners that tell
 the students something is there.  Use it to cover a solution until the
 student has tried, or a part of the exercise for later.  Rest the pen on
 it to move it, near a corner to resize it; a tap of the eraser removes
-it and shows what was covered.  Nothing under a Hide is lost, and a
+it and shows what was covered, and **Undo** takes back any of these as
+it does your writing.  Nothing under a Hide is lost, and a
 student cannot uncover it by hiding your writing on their screen: a Hide
 belongs to the page, not to your writing.  A Hide is white in an
 exported picture and in the page file as other programs see it.