Add Vertical and Horizontal perspective sliders to Compose

The keystone existed in framing but nothing in develop could reach it:
framing is presented by its own Compose panel rather than generated, so
new framing parameters get no control until the panel names them.

Compose now has Vertical and Horizontal sliders under Straighten,
mirrored from the session like the angle, recorded as parameter steps
("Vertical Perspective" in the history), cleared by the Compose reset
and by opening the next photograph. Releasing either slider refits the
crop the way releasing the straighten slider does: a keystone alone
needs no crop, but it moves the empty corners of a straightened frame,
so the crop that avoided them before may not after, or may have room
to grow back.

ui/dr-ui/src/labels.rs

@@ -274,6 +274,12 @@ fn catalogued(key: &str) -> Option<&'static str> {
         "param.crop_y" => "Crop Top",
         "param.crop_w" => "Crop Width",
         "param.crop_h" => "Crop Height",
+        // TRACES: FR-DEV-20
+        // The panel labels these "Vertical" and "Horizontal" under the
+        // straightening, where the context says what they correct; a history
+        // row has no such context, so it says it.
+        "param.keystone_v" => "Vertical Perspective",
+        "param.keystone_h" => "Horizontal Perspective",
 
         // TRACES: FR-DEV-5
         // The steps that are not a parameter moving.