Put a scrollbar on the library grid

The timeline beside the grid says *when* the view is. It does not say
how far through the library the view is, and scrubbing it jumps by
date. The bar gives the plain desktop answer: a thumb in proportion to
one screen of the whole grid, dragged or paged by clicking the track.
It reads the viewport that the rows are counted into, so it spans every
photograph and not only the loaded window of cells.

The Flickable keeps its id, its `interactive` arbitration with the
hold-to-pick-up and its pinch and zoom catchers. It now fills a
Rectangle that takes its place, and its stretch, in the grid's layout.

docs/gestures.md

@@ -700,7 +700,7 @@ The cursor selects what it lands on, so walking and judging are one hand's work.
 
 There is no wheel on a tablet, so without the pinch the cell size could only be changed by a control a finger cannot reach.
 
-<sub>`ui/dr-ui/ui/library.slint:3526`</sub>
+<sub>`ui/dr-ui/ui/library.slint:3530`</sub>
 
 ### File photographs in a collection
 
@@ -710,7 +710,7 @@ There is no wheel on a tablet, so without the pinch the cell size could only be
 
 The selection is what the drag carries, which is why selecting several is worth the mode: forty photographs file in one gesture.
 
-<sub>`ui/dr-ui/ui/library.slint:3725`</sub>
+<sub>`ui/dr-ui/ui/library.slint:3729`</sub>
 
 ### Open a photograph
 
@@ -721,7 +721,7 @@ The selection is what the drag carries, which is why selecting several is worth
 
 A tap opens; a tap that *moved* does not. Travel is what separates a deliberate tap from a hand brushing past, and it is the only thing that does: the two are the same length. An earlier version required the finger to dwell 120 ms instead, and that rejected ordinary taps — a real tap is often quicker than a brush.
 
-<sub>`ui/dr-ui/ui/library.slint:4030`</sub>
+<sub>`ui/dr-ui/ui/library.slint:4034`</sub>
 
 ### Rate a photograph without opening it
 
@@ -732,7 +732,7 @@ A tap opens; a tap that *moved* does not. Travel is what separates a deliberate
 
 A star has to take the press without it also reaching the cell, or every rating throws the user into develop.
 
-<sub>`ui/dr-ui/ui/library.slint:4153`</sub>
+<sub>`ui/dr-ui/ui/library.slint:4157`</sub>
 
 ### Choose the frame a folded burst shows
 
@@ -742,7 +742,7 @@ A star has to take the press without it also reaching the cell, or every rating
 
 A folded burst draws its earliest frame, which is a fact about the clock and not a judgement about the photograph — nothing in this application ranks a frame (FR-CULL-5). But the point of a burst is that one of the twelve is better than the other eleven, and the photographer is the only one who knows which. So the choice is offered on the frames themselves, while they are open and side by side, which is the one moment the alternatives are on screen to be compared.
 
-<sub>`ui/dr-ui/ui/library.slint:4286`</sub>
+<sub>`ui/dr-ui/ui/library.slint:4290`</sub>
 
 ### Drop the selection but keep selecting
 
@@ -753,7 +753,7 @@ A folded burst draws its earliest frame, which is a fact about the clock and not
 
 Distinct from Done, which leaves the mode entirely. Clearing keeps it, so the next selection can start straight away.
 
-<sub>`ui/dr-ui/ui/library.slint:4963`</sub>
+<sub>`ui/dr-ui/ui/library.slint:4981`</sub>
 
 ### Select everything the grid is showing
 
@@ -764,7 +764,7 @@ Distinct from Done, which leaves the mode entirely. Clearing keeps it, so the ne
 
 A scoped grid of two hundred frames is two hundred taps otherwise, and "all of them, except those three" is a far more common shape than the taps it took to say it.
 
-<sub>`ui/dr-ui/ui/library.slint:4982`</sub>
+<sub>`ui/dr-ui/ui/library.slint:5000`</sub>
 
 ### Take photographs out of a collection
 
@@ -774,7 +774,7 @@ A scoped grid of two hundred frames is two hundred taps otherwise, and "all of t
 
 The badge on a cell says a photograph is filed in three collections and never which. This is the sheet that names them, and the only way out of one the grid is not currently scoped to.
 
-<sub>`ui/dr-ui/ui/library.slint:5163`</sub>
+<sub>`ui/dr-ui/ui/library.slint:5181`</sub>
 
 ## Settings