Bind the two develop gestures that were described and not bound

FR-DEV-16's book said resetting a control and hiding a mask layer were
reachable by pointer and by finger, and stopped there. The reason was
honest: the generated rows have no focus, so "reset the focused control"
named a thing the panel could not point at. But a photographer at the
keyboard means something narrower than focus. They mean the slider they
just dragged too far, and that is a thing the panel can remember.

So the Adjustments global keeps the last control moved — two indices,
written where the panel forwards the change and cleared when the next
photograph opens, so a reset cannot reach back into the previous edit
through an index that happens to be shared. R puts it back, through the
same callback the track's double-click takes, and is silent until
something has moved.

The mask layer needs no such notion, because the panel already has a
selection: the rows the edge controls point at. H hides or shows those,
through the path the ring at the head of the row takes, so it is an edit
and a history step exactly as the ring is. A mixed selection goes to
shown, since the layer nobody can see is the one being asked about.

Both tags now carry the key, and the book says so.

ui/dr-ui/ui/masks.slint

@@ -213,6 +213,7 @@ component MaskEntry inherits Rectangle {
             //   why:     Disabling a layer is the before-and-after a local
             //            edit constantly wants, so it is one press away rather
             //            than inside the row. It is an edit and does take a
+            //   keys:    H, for the selected layer
             //            history step, unlike holding "Before" — the layer
             //            really is off until it is switched back on.
             //
@@ -819,6 +820,12 @@ export global Masking {
     callback add-range(bool);
     callback add-subject(int);
     callback add-category(string);
+    /// TRACES: FR-DEV-16
+    /// The same switch for whichever layers are selected, from the keyboard.
+    /// No id, because the selection is the session's and this file cannot
+    /// see it; Rust answers by flipping each selected layer through the same
+    /// path the ring takes, so it is one history step per layer either way.
+    callback selected-toggled();
 }
 
 export component MaskPanel inherits Rectangle {