Make straighten/orthogonalize act like a single operation

- Display only one or the other in the radial menu
- Rename "Orthogonalize" to "Square"
- Use "S" as shortcut for both

Refs #1839

data/core.yaml

@@ -56,20 +56,20 @@ en:
         area: Made an area circular.
       not_closed: This can't be made circular because it's not a loop.
     orthogonalize:
-      title: Orthogonalize
-      description: Square these corners.
-      key: Q
+      title: Square
+      description:
+        line: Square the corners of this line.
+        area: Square the corners of this area.
+      key: S
       annotation:
         line: Squared the corners of a line.
         area: Squared the corners of an area.
-      not_closed: This can't be made square because it's not a loop.
     straighten:
       title: Straighten
       description: Straighten this line.
       key: S
-      annotation: Straightened the line.
-      is_closed: This can't be straightened because it's a loop.
-      too_bendy: This can't be straightened because it's too bendy.
+      annotation: Straightened a line.
+      too_bendy: This can't be straightened because it bends too much.
     delete:
       title: Delete
       description: Remove this from the map.