Link fifty gestures on the help sheet to the manual section that shows them

The sheet could now offer "See it", but no gesture said where to look.

Every GESTURE tag whose move the manual describes names that section:
the white-balance picker, zoom and pan, masks, undo and snapshots,
export, colour labels and ratings, selection, collections, the People
page, thumbnail size. Fifty of the fifty-one; the one left, putting a
single control back to its default, has no section and is too small to
earn one.

Three important gestures had nowhere to land, so the manual gains three
short sections, without pictures for now: Bursts (opening a folded
burst, choosing the frame it shows, and the eyes-open filter), Moving
between photographs (the roll, the arrows and A/D in develop) and
Copying settings (Copy, Paste, Paste to N, and choosing what a copy
carries). The page, the gesture book and docs/gestures.md are
regenerated from them.

ui/dr-ui/ui/history.slint

@@ -98,6 +98,7 @@ component SnapshotItem inherits Rectangle {
         //            I liked twenty minutes ago" is how a choice between two
         //            treatments is actually made. It takes no history step
         //            and changes nothing; letting go puts the edit back.
+        //   manual:  history-snapshots-presets
         //
         // TRACES: FR-DEV-7
         eye := TouchArea {
@@ -314,6 +315,7 @@ export component HistoryPanel inherits Rectangle {
         //            of the edit, so it survives a restart and reaches the
         //            other device. Pressing a snapshot puts the photograph
         //            back to it, as one step that undo takes back whole.
+        //   manual:  history-snapshots-presets
         if Develop.enabled: HorizontalLayout {
             spacing: Theme.gap-sm;