Show and set colour labels in the grid and develop, and filter by them

Colour labels could be read from a Lightroom sidecar and queried by the
selector, but nothing drew one or set one, so the only labels a library
held were ones another program had written.

Every mark carries its label's initial on its colour — R, Y, G, B, P —
so a label is read without telling red from green, which is what
NFR-A11Y-3 asks of colour labels by name. A grid cell shows the mark
before its filename. In the grid, 6, 7, 8 and 9 set red, yellow, green and
blue as Lightroom's keys do, on the photograph under the pointer or on
the selection by the rule the star keys follow; the same key again takes
the label off, and over a mixed selection it sets it on all. The
selection bar gains Label, which opens the six choices — each a mark and
a name — and purple, which has no key, is there. In develop the top bar
says "Label: Green" beside the mark, opens the same choices, and 6-9
label the open photograph.

Each gesture is one catalog transaction, then the grid, the counts and
both sidecars are written as a rating's are. The filter bar gains a chip
per label, its mark and its name with a count, one at a time; the filter
is one SQL term, travels in the place record, and "All" clears it.

ui/dr-ui/src/gesture_book.rs

@@ -113,6 +113,13 @@ pub const GESTURES: &[Gesture] = &[
         pointer: "Double-click it, or press the zoom readout floating over the canvas",
         keys: "Z",
     },
+    Gesture {
+        title: "Give this photograph a colour label",
+        section: "Develop",
+        touch: "Tap Label in the top bar, then a colour",
+        pointer: "Click Label in the top bar, then a colour",
+        keys: "6 red, 7 yellow, 8 green, 9 blue; the same key again takes it off",
+    },
     Gesture {
         title: "Move to the next or previous photograph",
         section: "Develop",
@@ -281,6 +288,13 @@ pub const GESTURES: &[Gesture] = &[
         pointer: "Open the People chip on the filter bar, click each name, then switch the chip beside them to \"all of them\"",
         keys: "",
     },
+    Gesture {
+        title: "Show only photographs with one colour label",
+        section: "Library grid",
+        touch: "Tap its chip in the filter bar",
+        pointer: "Click its chip in the filter bar",
+        keys: "",
+    },
     Gesture {
         title: "Export the selection as the last export was",
         section: "Library grid",
@@ -302,6 +316,13 @@ pub const GESTURES: &[Gesture] = &[
         pointer: "Click a star chip in the filter bar",
         keys: "Hold F and tap a digit for exactly that many stars, or two digits for everything between them; tap F alone to show every rating again",
     },
+    Gesture {
+        title: "Give photographs a colour label",
+        section: "Library grid",
+        touch: "Select them, then Label in the selection bar and tap a colour",
+        pointer: "Select them, then Label in the selection bar and click a colour",
+        keys: "6 red, 7 yellow, 8 green, 9 blue, with the pointer over it or on the selection; the same key again takes the label off",
+    },
     Gesture {
         title: "Resize the thumbnails",
         section: "Library grid",