Open Help from develop as well as from the grid

The "Controls and shortcuts" sheet was drawn by LibraryGrid, so only the
grid's Help button and its F1 could open it. Develop, where most of the
keys it lists are bound (Ctrl+E, Ctrl+Shift+C, A/D, Z, R, H, [ ]), had no
way to it: a photographer who wanted to look a shortcut up had to leave
the photograph they wanted it for.

The sheet now hangs off the shell beside the export and copy sheets, on a
`help-open` property both views set. The grid's Help button and F1 raise
it through a callback, and its keys stand down through the `sheet-open`
they already honour for the export sheet, so Escape falls through to the
shell, which closes it. Develop gains a Help button beside Settings in
its top bar, as in the library header, and F1 in its key scope; its keys
decline while the sheet is up, as they do for the other two sheets, so
nothing behind it is rated or stepped. The book already begins with the
Develop section, so from develop it opens where the reader wants it.

Tests hold the shape: the sheet is drawn by the shell and not the grid,
and develop's opening guard names all three sheets and its F1 opens this
one. The new GESTURE: block puts the develop route in the book.

ui/dr-ui/ui/develop.slint

@@ -63,6 +63,9 @@ export component StatusBar inherits Rectangle {
     callback back-to-library();
     callback open-people();
     callback open-settings();
+    /// TRACES: FR-UI-4
+    /// Open the gesture reference, which the window draws.
+    callback open-help();
     callback toggle-panel();
     callback export-image();
     callback undo();
@@ -303,6 +306,17 @@ export component StatusBar inherits Rectangle {
             clicked => { root.export-image(); }
         }
 
+        // TRACES: FR-UI-4
+        // The pointer and touch way to the reference F1 opens. Beside
+        // Settings, as in the library header, because it is about the
+        // application rather than about the photograph — and in the same
+        // place on both screens, so it is found where it was last seen.
+        Button {
+            text: "Help";
+            y: (parent.height - self.height) / 2;
+            clicked => { root.open-help(); }
+        }
+
         // Reachable from develop as well as from the grid: export defaults are
         // most likely to be wanted with a finished photograph on screen, which
         // is exactly where this bar is and the library header is not.