Open the film list as a popup, so every stock can be scrolled to

The open stock list showed ten rows, None to Kodak Kodachrome 64, and
the other eighteen - all seven black-and-white stocks among them - could
not be reached. The data was whole; the list could not be scrolled.

Reproduced on the manual rig (Xvfb, xdotool, the automation hook):

- a drag on the list scrolled the develop column, never the list;
- a wheel run over the list scrolled the column past it, whenever the
  column had scrolled under that pointer in the last 800 ms - which is
  how the list is reached, by wheeling the column down to it. After a
  pause and a pointer move the wheel did reach the list;
- no key did anything.

The cause is Slint's routing, not the list. Since 2d878c2 the list was a
Flickable inside the develop column's Flickable, and Slint offers every
pointer event to the outermost Flickable first
(input_event_filter_before_children, i-slint-core 1.17.1 flickable.rs).
The column holds a press back (DelayForwarding) and intercepts the first
move past 8 px on the axis it can scroll, so the list never saw a drag.
For the wheel it intercepts while its own last wheel event is under
800 ms old and within 2 px, and always for a touchpad gesture that opens
with TouchPhase::Started - so on a touchpad the list could get no wheel
at all.

The list is now a PopupWindow under the Film row. A popup is its own
item tree: while it is open, events go to it and to nothing beneath it,
so the list scrolls by wheel, drag and flick however the panel is nested
and whatever the column did last. The alternative, standing the column
down while the pointer is over the list (the sliders' hover trick), fixes
the drag but not the wheel - `interactive: false` does not gate wheel
interception - so it would have left the bug for touchpad users.

The reason 2d878c2 bounded the list still holds: it is at most 320 px
and never lengthens the column, and now it covers the sliders instead of
pushing them down. The column cannot be scrolled while it is open, which
suits a one-click question; it closes on choosing, on Escape or Back, or
on a press outside it.

Keys, with the list open: Up and Down move along it from the chosen
stock and scroll it into view, Enter chooses, Escape or Back closes it
unchanged. A popup is its own focus tree, so the keys are taken when it
opens and Slint returns focus to the develop view's scope when it closes.
The Film row gains a button role, so a screen reader and the automation
hook can name it.

ui/dr-ui/src/gesture_book.rs

@@ -32,6 +32,14 @@ pub const GESTURES: &[Gesture] = &[
         keys: "",
         manual: "white-balance-from-the-photograph",
     },
+    Gesture {
+        title: "Choose a film stock from the keyboard",
+        section: "Develop",
+        touch: "Tap the Film row, flick the list, tap a stock",
+        pointer: "Click the Film row, then a stock; the wheel or a drag scrolls the list",
+        keys: "With the list open, ↑ and ↓ move along it, Enter chooses, and Escape or Back closes it unchanged",
+        manual: "film",
+    },
     Gesture {
         title: "Magnify the photograph by any amount",
         section: "Develop",