Make develop's Help a "?" beside Settings, so Settings fits at 1600

Adding a labelled Help button to develop's top bar made the strip about
100 pixels wider than a 1600-pixel window. The strip scrolls, so nothing
became unreachable, but Settings was off the right-hand end until the
bar was dragged.

Help is now a square IconButton with a drawn question mark (a new "help"
icon, drawn rather than typed for the reason icons.slint gives about
Android's fonts), and screen readers still hear it as "Help". That saves
60 pixels, which was not enough alone: the strip had fit with 3 to spare
before. The rest comes from the spacing. Controls that belong together
now sit in groups at gap-sm with gap between groups: Pick and Reject,
Undo and Redo, Copy, Paste and Presets, and "?" and Settings. The empty
export-status caption no longer takes a slot and two spacings while it
has nothing to say. At 1600 wide with the panel open the whole bar now
shows, Settings included.

The grid's header keeps its worded Help button; it has room for it.
The develop "Open this list" gesture now names the "?" button, and the
gesture book is regenerated from it.

docs/gestures.md

@@ -53,8 +53,8 @@ A model's mask stops inside a shoulder and leaks into the hair, and no single ed
 
 ### Open this list
 
-- **Touch** — Press Help in the top bar, and Done to put it away
-- **Pointer** — Press Help in the top bar, and Done to put it away
+- **Touch** — Press "?" in the top bar, beside Settings, and Done to put it away
+- **Pointer** — Press "?" in the top bar, beside Settings, and Done to put it away
 - **Keyboard** — `F1`; the key that closes any sheet puts it away
 
 Most of the keys are develop's, and a reference that could only be opened from the grid had to be looked up before opening the photograph they were wanted for.