Step along the roll by library ordinal, and keep its mark on the open photograph

In a library's develop view the arrows, space, A and D opened
`library-roll-pick(library-roll-current ± 1)`. `roll-current` is a row
of the loaded window, set when a photograph was opened and never again.
Two things followed on any library larger than one window:

- Holding D stopped dead at the end of the loaded window: the pick of a
  row past `ctl.paths` found no path and did nothing, a few screenfuls
  into a library of thousands. A stopped at its start the same way.
- Any reload of the window — a background sync, a judgement that drops
  the open frame out of a filter — left `roll-current` on a row that now
  held another photograph. The roll marked it, develop's rating keys
  judged it, and the next step walked on from it.

The keys now call `library-roll-step(delta)`, and Rust steps as
`move_cursor` does in the grid: from the open photograph's ordinal
(`index`, which `report_position` already keeps), clamped to the
library, loading the window around the target only when it lies
outside. The window-bringing half of `place_cursor` is shared for this
as `bring_window_to`, so the grid cursor and the roll move the window
the same way. Catalog reads stay proportional to the window: one
`load_window` per window crossed, none per step within it.

The open photograph is also remembered by image id. Every
`load_window` finds it again in the rows it just read (a scan of one
window, no query), puts `roll-current` and `index` back on it, or takes
the mark off when it is not there. When it is missing but its ordinal
is still inside the window, it has left the grid and its successor
moved up into its place, so the next step forward lands on that ordinal
instead of skipping the successor.

A click on the roll still reports a row; it is right because the cells
it is drawn from and `ctl.paths` are replaced together, and it now goes
through the same open path as a step.

Fixes #64.

ui/dr-ui/src/library_ui/timeline.rs

@@ -18,7 +18,7 @@ use crate::{AppWindow, Library, TimelineBar, View};
 
 use super::controller::LibraryController;
 use super::filter_bar::{push_people_chips, refilter};
-use super::window::{load_window, window_start};
+use super::window::{load_window, mark_open, window_start};
 
 /// Move the timeline's zoom by whole levels.
 ///
@@ -814,9 +814,7 @@ pub(crate) fn apply_place(
                 window.global::<Library>().set_library_roll_centre(true);
                 let offset = *ctl.offset.borrow();
                 if let Some(row) = at.checked_sub(offset) {
-                    window
-                        .global::<Library>()
-                        .set_library_roll_current(row as i32);
+                    mark_open(window, ctl, row);
                 }
                 open(place.path.clone());
                 // After the open, which resets the readout to "1 of 1" on its