Show a thin scroll cue on Android instead of no scrollbar at all

On Android the desktop scrollbar is off, so every scroller that has one
on a desktop (the develop column, the grid, the collections sidebar,
Settings, the help sheet and the film list) gave no sign of how long it
was or where the view was in it. That is how the black-and-white film
stocks came to look deleted when the list could not scroll.

ScrollBar now has a second mode, chosen in the one Scrolling global:
where bars are off and `cue` is on, it draws the thumb alone, 3 px wide
against the edge, while the viewport moves, and fades it 500 ms after
the last move. It has no TouchArea, so a flick that starts on it
scrolls the content. Rust sets cue on touch-first builds; a desktop
build shows it when DR_SCROLL_CUE is set, to look at it without a
device. Desktop is otherwise unchanged.

Test: on the testing backend's help sheet, a press on the cue moves
nothing, a drag starting on it carries the list with the finger, the
cue is drawn while the list moves (drag, fling, wheel) and not once it
is idle; with bars on there is no cue and the thumb still takes a drag.

docs/gestures.md

@@ -354,7 +354,7 @@ One key for "up one", innermost first: a question before the sheet under it, a s
 
 A list cut off at its edge looks, to a mouse, like a list that ends there — the film stocks past the tenth read as deleted. The bar says there is more and where the view is in it, and it is the one way to scroll that needs neither a wheel nor a drag on the content, which may be a row that would take the click.
 
-<sub>`ui/dr-ui/ui/widgets.slint:1131`</sub>
+<sub>`ui/dr-ui/ui/widgets.slint:1138`</sub>
 
 ## Collections sidebar