Swipe up in develop for the photo roll

Develop opens one photograph. The grid handed over a path and nothing
else, so `index` and `total` were pinned to "1 of 1" on the way in and the
only route to the next frame was back to the library, find your place,
tap again. Fine once; intolerable through a set of forty, which is the
situation the develop view exists for.

The roll is the grid's already-loaded window along the foot of the
canvas. Swipe up to bring it out, swipe down to put it away — the sheet
gesture, already in the hands of anyone who has used a phone — and a
handle is drawn at the edge so the gesture is discoverable rather than
folklore, and so a pointer, which has no swipe to make, has a way in.

`SwipeGestureHandler` wraps the strip rather than sitting over or under
it, which is what it is built for: it delays a press the way a Flickable
does, forwards it to the children if no swipe develops and claims it once
one does, so a tap reaches the thumbnail and a drag does not. It covers
only the band along the bottom — above that, a drag still belongs to the
photograph, for panning and for the crop.

Picking goes through the same path a cell click does, so the outgoing
edit is persisted before the next image loads. The strip marks what is
open and scrolls to keep the mark in view.

The position readout now says where in the *library* the open photograph
sits rather than "1 of 1". Set after the open rather than before, since
the generic open path resets it — and given as the library ordinal, not
the row in the loaded window, which is an artefact of how much has been
paged in and would jump about as the window moves.

ui/dr-ui/ui/app.slint

@@ -1,7 +1,7 @@
 import { Theme } from "theme.slint";
 import { AdjustPanel, GeometryPanel, ParamRow, TransferPanel } from "adjust.slint";
 import { LaunchScreen } from "launch.slint";
-import { LibraryGrid, LibraryCell, TimelineBar } from "library.slint";
+import { LibraryGrid, LibraryCell, TimelineBar, PhotoRoll } from "library.slint";
 import { Button, PanelHeading, Label, Value, Caption, Panel, EmptyState, ProgressBar, ActivityRow } from "widgets.slint";
 import { CollectionsPanel, CollectionRow, OfflinePrompt } from "collections.slint";
 import { HistogramPanel, HistogramView } from "histogram.slint";
@@ -510,6 +510,15 @@ export component AppWindow inherits Window {
     callback library-zoom-cells(int);
     in property <int> library-scroll-to: 0;
     in property <int> library-scroll-token: 0;
+    /// Which row of the loaded window is the photograph currently open in
+    /// develop, so the roll can mark it. `-1` when it is not in the window.
+    in property <int> library-roll-current: -1;
+    /// Whether the photo roll is out. Owned here rather than by the roll, so
+    /// it survives the develop view being torn down and rebuilt.
+    in-out property <bool> roll-open: false;
+    /// A photograph was chosen from the roll: the row within the loaded
+    /// window, which is what a cell click reports too.
+    callback library-roll-pick(int);
     callback library-sync-now();
     callback library-columns-changed();
     callback library-scrolled(int);
@@ -1636,6 +1645,24 @@ in property <bool> panel-visible: true;
                         }
                     }
 
+                    // The photo roll, along the foot of the canvas.
+                    //
+                    // Only where a library was opened: with files named on the
+                    // command line there is no window of photographs to roll
+                    // through, which is the same condition the "‹ Library"
+                    // button uses.
+                    //
+                    // Last in the canvas, so it draws over the photograph
+                    // rather than under it.
+                    if root.library-total > 0: PhotoRoll {
+                        width: 100%;
+                        height: 100%;
+                        cells: root.library-cells;
+                        current: root.library-roll-current;
+                        open <=> root.roll-open;
+                        pick(i) => { root.library-roll-pick(i); }
+                    }
+
                     // Report size changes so the render target can be resized to
                     // match. Width and height are tracked separately because
                     // Slint has no single "geometry changed" hook.