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/collections.slint

@@ -342,6 +342,7 @@ component TreeRow inherits Rectangle {
     //            something, and it is what every mobile list already uses to
     //            pick a row up. The row lifts the moment it fires, so the
     //            gesture says it has been understood before anything moves.
+    //   manual:  collections
     //
     // GESTURE: Act on a collection — rename, nest, un-nest, delete
     //   where:   Collections sidebar
@@ -352,6 +353,7 @@ component TreeRow inherits Rectangle {
     //            uses. One menu for everything done to a row, because there is
     //            one hold per row: while the hold opened the offline question
     //            by itself, nothing else the tree can do had a touch route.
+    //   manual:  collections
     //
     // GESTURE: Take a collection back out of the one it is nested in
     //   where:   Collections sidebar
@@ -365,6 +367,7 @@ component TreeRow inherits Rectangle {
     //            library — and wrong for a collection, which has a top level
     //            to be returned to. Without it a collection dragged into
     //            another was in there permanently.
+    //   manual:  collections
     //
     // GESTURE: Rename a collection
     //   where:   Collections sidebar
@@ -373,6 +376,7 @@ component TreeRow inherits Rectangle {
     //   why:     Double-click is what a file manager and a Lightroom panel use
     //            for the same thing, so it needs no discovering — but nothing
     //            on screen says so, which is what the menu item is for.
+    //   manual:  collections
     touch := TouchArea {
         // Disabled while the field is up, so a click landing on the row rather
         // than inside the input does not re-select the collection out from