Close the help sheet with Escape, and keep keys from acting behind it

F1 opens the help sheet from the grid, and nothing on the keyboard closed
it: Escape fell through to the shell, and every other key went on judging,
labelling and keywording the photographs hidden behind the sheet.

Escape and Back now close the sheet first, ahead of the grid's other
sheets, since it is drawn over all of them. While it is open the grid's
handler declines every other key, so a stray P or Ctrl+K changes nothing
the reader cannot see.

ui/dr-ui/src/gesture_book.rs

@@ -475,9 +475,9 @@ pub const GESTURES: &[Gesture] = &[
     Gesture {
         title: "Open this list",
         section: "Library grid",
-        touch: "Press Help in the header",
-        pointer: "Press Help in the header",
-        keys: "F1",
+        touch: "Press Help in the header, and Done to put it away",
+        pointer: "Press Help in the header, and Done to put it away",
+        keys: "F1, and Escape to put it away",
         manual: "",
     },
     Gesture {